Remove 100s of CPU time (10%) from build times (1465s -> 1302s)

I saw that boost/lexical_cast was costing about 100s in CPU time on our
compiles. We can fix this trivially by doing explicit template
instantiation in exactly one place and eliminating all other includes of
it, which is a code improvement anyway by hiding the boost.

Before:
```
lix/lix2 » ClangBuildAnalyzer --analyze buildtimeold.bin
Analyzing build trace from 'buildtimeold.bin'...
**** Time summary:
Compilation (551 times):
  Parsing (frontend):         1465.3 s
  Codegen & opts (backend):   1110.9 s

<snip>

**** Expensive headers:
178153 ms: ../src/libcmd/installable-value.hh (included 52 times, avg 3426 ms), included via:
  40x: command.hh
  5x: command-installable-value.hh
  3x: installable-flake.hh
  2x: <direct include>
  2x: installable-attr-path.hh

176217 ms: ../src/libutil/error.hh (included 246 times, avg 716 ms), included via:
  36x: command.hh installable-value.hh installables.hh derived-path.hh config.hh experimental-features.hh
  12x: globals.hh config.hh experimental-features.hh
  11x: file-system.hh file-descriptor.hh
  6x: serialise.hh strings.hh
  6x: <direct include>
  6x: archive.hh serialise.hh strings.hh
  ...

173243 ms: ../src/libstore/store-api.hh (included 152 times, avg 1139 ms), included via:
  55x: <direct include>
  39x: command.hh installable-value.hh installables.hh
  7x: libexpr.hh
  4x: local-store.hh
  4x: command-installable-value.hh installable-value.hh installables.hh
  3x: binary-cache-store.hh
  ...

170482 ms: ../src/libutil/serialise.hh (included 201 times, avg 848 ms), included via:
  37x: command.hh installable-value.hh installables.hh built-path.hh realisation.hh hash.hh
  14x: store-api.hh nar-info.hh hash.hh
  11x: <direct include>
  7x: primops.hh eval.hh attr-set.hh nixexpr.hh value.hh source-path.hh archive.hh
  7x: libexpr.hh value.hh source-path.hh archive.hh
  6x: fetchers.hh hash.hh
  ...

169397 ms: ../src/libcmd/installables.hh (included 53 times, avg 3196 ms), included via:
  40x: command.hh installable-value.hh
  5x: command-installable-value.hh installable-value.hh
  3x: installable-flake.hh installable-value.hh
  2x: <direct include>
  1x: installable-derived-path.hh
  1x: installable-value.hh
  ...

159740 ms: ../src/libutil/strings.hh (included 221 times, avg 722 ms), included via:
  37x: command.hh installable-value.hh installables.hh built-path.hh realisation.hh hash.hh serialise.hh
  19x: <direct include>
  14x: store-api.hh nar-info.hh hash.hh serialise.hh
  11x: serialise.hh
  7x: primops.hh eval.hh attr-set.hh nixexpr.hh value.hh source-path.hh archive.hh serialise.hh
  7x: libexpr.hh value.hh source-path.hh archive.hh serialise.hh
  ...

156796 ms: ../src/libcmd/command.hh (included 51 times, avg 3074 ms), included via:
  42x: <direct include>
  7x: command-installable-value.hh
  2x: installable-attr-path.hh

150392 ms: ../src/libutil/types.hh (included 251 times, avg 599 ms), included via:
  36x: command.hh installable-value.hh installables.hh path.hh
  11x: file-system.hh
  10x: globals.hh
  6x: fetchers.hh
  6x: serialise.hh strings.hh error.hh
  5x: archive.hh
  ...

133101 ms: /nix/store/644b90j1vms44nr18yw3520pzkrg4dd1-boost-1.81.0-dev/include/boost/lexical_cast.hpp (included 226 times, avg 588 ms), included via
:
  37x: command.hh installable-value.hh installables.hh built-path.hh realisation.hh hash.hh serialise.hh strings.hh
  19x: file-system.hh
  11x: store-api.hh nar-info.hh hash.hh serialise.hh strings.hh
  7x: primops.hh eval.hh attr-set.hh nixexpr.hh value.hh source-path.hh archive.hh serialise.hh strings.hh
  7x: libexpr.hh value.hh source-path.hh archive.hh serialise.hh strings.hh
  6x: eval.hh attr-set.hh nixexpr.hh value.hh source-path.hh archive.hh serialise.hh strings.hh
  ...

132887 ms: /nix/store/h2abv2l8irqj942i5rq9wbrj42kbsh5y-gcc-12.3.0/include/c++/12.3.0/memory (included 262 times, avg 507 ms), included via:
  36x: command.hh installable-value.hh installables.hh path.hh types.hh ref.hh
  16x: gtest.h
  11x: file-system.hh types.hh ref.hh
  10x: globals.hh types.hh ref.hh
  10x: json.hpp
  6x: serialise.hh
  ...

  done in 0.6s.
```

After:
```
lix/lix2 » maintainers/buildtime_report.sh build
Processing all files and saving to '/home/jade/lix/lix2/maintainers/../buildtime.bin'...
  done in 0.6s. Run 'ClangBuildAnalyzer --analyze /home/jade/lix/lix2/maintainers/../buildtime.bin' to analyze it.
Analyzing build trace from '/home/jade/lix/lix2/maintainers/../buildtime.bin'...
**** Time summary:
Compilation (551 times):
  Parsing (frontend):         1302.1 s
  Codegen & opts (backend):    956.3 s

<snip>

**** Expensive headers:
178145 ms: ../src/libutil/error.hh (included 246 times, avg 724 ms), included via:
  36x: command.hh installable-value.hh installables.hh derived-path.hh config.hh experimental-features.hh
  12x: globals.hh config.hh experimental-features.hh
  11x: file-system.hh file-descriptor.hh
  6x: <direct include>
  6x: serialise.hh strings.hh
  6x: fetchers.hh hash.hh serialise.hh strings.hh
  ...

154043 ms: ../src/libcmd/installable-value.hh (included 52 times, avg 2962 ms), included via:
  40x: command.hh
  5x: command-installable-value.hh
  3x: installable-flake.hh
  2x: <direct include>
  2x: installable-attr-path.hh

153593 ms: ../src/libstore/store-api.hh (included 152 times, avg 1010 ms), included via:
  55x: <direct include>
  39x: command.hh installable-value.hh installables.hh
  7x: libexpr.hh
  4x: local-store.hh
  4x: command-installable-value.hh installable-value.hh installables.hh
  3x: binary-cache-store.hh
  ...

149948 ms: ../src/libutil/types.hh (included 251 times, avg 597 ms), included via:
  36x: command.hh installable-value.hh installables.hh path.hh
  11x: file-system.hh
  10x: globals.hh
  6x: fetchers.hh
  6x: serialise.hh strings.hh error.hh
  5x: archive.hh
  ...

144560 ms: ../src/libcmd/installables.hh (included 53 times, avg 2727 ms), included via:
  40x: command.hh installable-value.hh
  5x: command-installable-value.hh installable-value.hh
  3x: installable-flake.hh installable-value.hh
  2x: <direct include>
  1x: installable-value.hh
  1x: installable-derived-path.hh
  ...

136585 ms: ../src/libcmd/command.hh (included 51 times, avg 2678 ms), included via:
  42x: <direct include>
  7x: command-installable-value.hh
  2x: installable-attr-path.hh

133394 ms: /nix/store/h2abv2l8irqj942i5rq9wbrj42kbsh5y-gcc-12.3.0/include/c++/12.3.0/memory (included 262 times, avg 509 ms), included via:
  36x: command.hh installable-value.hh installables.hh path.hh types.hh ref.hh
  16x: gtest.h
  11x: file-system.hh types.hh ref.hh
  10x: globals.hh types.hh ref.hh
  10x: json.hpp
  6x: serialise.hh
  ...

89315 ms: ../src/libstore/derived-path.hh (included 178 times, avg 501 ms), included via:
  37x: command.hh installable-value.hh installables.hh
  25x: store-api.hh realisation.hh
  7x: primops.hh eval.hh attr-set.hh nixexpr.hh value.hh context.hh
  6x: eval.hh attr-set.hh nixexpr.hh value.hh context.hh
  6x: libexpr.hh value.hh context.hh
  6x: shared.hh
  ...

87347 ms: /nix/store/h2abv2l8irqj942i5rq9wbrj42kbsh5y-gcc-12.3.0/include/c++/12.3.0/ostream (included 273 times, avg 319 ms), included via:
  35x: command.hh installable-value.hh installables.hh path.hh types.hh ref.hh memory unique_ptr.h
  12x: regex sstream istream
  10x: file-system.hh types.hh ref.hh memory unique_ptr.h
  10x: gtest.h memory unique_ptr.h
  10x: globals.hh types.hh ref.hh memory unique_ptr.h
  6x: fetchers.hh types.hh ref.hh memory unique_ptr.h
  ...

85249 ms: ../src/libutil/config.hh (included 213 times, avg 400 ms), included via:
  37x: command.hh installable-value.hh installables.hh derived-path.hh
  20x: globals.hh
  20x: logging.hh
  16x: store-api.hh logging.hh
  6x: <direct include>
  6x: eval.hh attr-set.hh nixexpr.hh value.hh context.hh derived-path.hh
  ...

  done in 0.5s.
```

Change-Id: I27f0a2d566db17832cd9be935f12efe7f95b92d0

1 files changed, 39 insertions, 0 deletions

diff --git a/src/libutil/strings.cc b/src/libutil/strings.cc
index 9cb319cce..947478481 100644
--- a/src/libutil/strings.cc
+++ b/src/libutil/strings.cc
@@ -1,4 +1,6 @@
 #include "strings.hh"
+#include <boost/lexical_cast.hpp>
+#include <stdint.h>
 
 namespace nix {
 
@@ -89,6 +91,43 @@ std::string Rewriter::operator()(std::string s)
     return s;
 }
 
+template<class N>
+std::optional<N> string2Int(const std::string_view s)
+{
+    if (s.substr(0, 1) == "-" && !std::numeric_limits<N>::is_signed)
+        return std::nullopt;
+    try {
+        return boost::lexical_cast<N>(s.data(), s.size());
+    } catch (const boost::bad_lexical_cast &) {
+        return std::nullopt;
+    }
+}
+
+// Explicitly instantiated in one place for faster compilation
+template std::optional<unsigned char>  string2Int<unsigned char>(const std::string_view s);
+template std::optional<unsigned short> string2Int<unsigned short>(const std::string_view s);
+template std::optional<unsigned int> string2Int<unsigned int>(const std::string_view s);
+template std::optional<unsigned long> string2Int<unsigned long>(const std::string_view s);
+template std::optional<unsigned long long> string2Int<unsigned long long>(const std::string_view s);
+template std::optional<signed char> string2Int<signed char>(const std::string_view s);
+template std::optional<signed short> string2Int<signed short>(const std::string_view s);
+template std::optional<signed int> string2Int<signed int>(const std::string_view s);
+template std::optional<signed long> string2Int<signed long>(const std::string_view s);
+template std::optional<signed long long> string2Int<signed long long>(const std::string_view s);
+
+template<class N>
+std::optional<N> string2Float(const std::string_view s)
+{
+    try {
+        return boost::lexical_cast<N>(s.data(), s.size());
+    } catch (const boost::bad_lexical_cast &) {
+        return std::nullopt;
+    }
+}
+
+template std::optional<double> string2Float<double>(const std::string_view s);
+template std::optional<float> string2Float<float>(const std::string_view s);
+
 
 std::string toLower(const std::string & s)
 {