Show function names in error messages

Functions in Nix are anonymous, but if they're assigned to a
variable/attribute, we can use the variable/attribute name in error
messages, e.g.

while evaluating `concatMapStrings' at `/nix/var/nix/profiles/per-user/root/channels/nixos/nixpkgs/pkgs/lib/strings.nix:18:25':
...

1 files changed, 9 insertions, 4 deletions

diff --git a/src/libexpr/symbol-table.hh b/src/libexpr/symbol-table.hh
index 143fc495b..ccdabb028 100644
--- a/src/libexpr/symbol-table.hh
+++ b/src/libexpr/symbol-table.hh
@@ -28,17 +28,17 @@ private:
 
 public:
     Symbol() : s(0) { };
-    
+
     bool operator == (const Symbol & s2) const
     {
         return s == s2.s;
     }
-    
+
     bool operator != (const Symbol & s2) const
     {
         return s != s2.s;
     }
-    
+
     bool operator < (const Symbol & s2) const
     {
         return s < s2.s;
@@ -49,6 +49,11 @@ public:
         return *s;
     }
 
+    bool set() const
+    {
+        return s;
+    }
+
     bool empty() const
     {
         return s->empty();
@@ -66,7 +71,7 @@ inline std::ostream & operator << (std::ostream & str, const Symbol & sym)
 class SymbolTable
 {
 private:
-#if HAVE_TR1_UNORDERED_SET 
+#if HAVE_TR1_UNORDERED_SET
     typedef std::tr1::unordered_set<string> Symbols;
 #else
     typedef std::set<string> Symbols;