defer formals duplicate check for incresed efficiency all round

if we defer the duplicate argument check for lambda formals we can use more
efficient data structures for the formals set, and we can get rid of the
duplication of formals names to boot. instead of a list of formals we've seen
and a set of names we'll keep a vector instead and run a sort+dupcheck step
before moving the parsed formals into a newly created lambda. this improves
performance on search and rebuild by ~1%, pure parsing gains more (about 4%).

this does reorder lambda arguments in the xml output, but the output is still
stable. this shouldn't be a problem since argument order is not semantically
important anyway.

 before

  nix search --no-eval-cache --offline ../nixpkgs hello
    Time (mean ± σ):      8.550 s ±  0.060 s    [User: 6.470 s, System: 1.664 s]
    Range (min … max):    8.435 s …  8.666 s    20 runs

  nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
    Time (mean ± σ):     346.7 ms ±   2.1 ms    [User: 312.4 ms, System: 34.2 ms]
    Range (min … max):   343.8 ms … 353.4 ms    20 runs

  nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
    Time (mean ± σ):      2.720 s ±  0.031 s    [User: 2.415 s, System: 0.231 s]
    Range (min … max):    2.662 s …  2.780 s    20 runs

 after

  nix search --no-eval-cache --offline ../nixpkgs hello
    Time (mean ± σ):      8.462 s ±  0.063 s    [User: 6.398 s, System: 1.661 s]
    Range (min … max):    8.339 s …  8.542 s    20 runs

  nix eval -f ../nixpkgs/pkgs/development/haskell-modules/hackage-packages.nix
    Time (mean ± σ):     329.1 ms ±   1.4 ms    [User: 296.8 ms, System: 32.3 ms]
    Range (min … max):   326.1 ms … 330.8 ms    20 runs

  nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'
    Time (mean ± σ):      2.687 s ±  0.035 s    [User: 2.392 s, System: 0.228 s]
    Range (min … max):    2.626 s …  2.754 s    20 runs

2 files changed, 2 insertions, 2 deletions

diff --git a/tests/check.nix b/tests/check.nix
index ec455ae2d..ed91ff845 100644
--- a/tests/check.nix
+++ b/tests/check.nix
@@ -50,6 +50,6 @@ with import ./config.nix;
 
   fetchurl = import <nix/fetchurl.nix> {
     url = "file://" + toString ./lang/eval-okay-xml.exp.xml;
-    sha256 = "0kg4sla7ihm8ijr8cb3117fhl99zrc2bwy1jrngsfmkh8bav4m0v";
+    sha256 = "sha256-behBlX+DQK/Pjvkuc8Tx68Jwi4E5v86wDq+ZLaHyhQE=";
   };
 }
diff --git a/tests/lang/eval-okay-xml.exp.xml b/tests/lang/eval-okay-xml.exp.xml
index 92b75e0b8..20099326c 100644
--- a/tests/lang/eval-okay-xml.exp.xml
+++ b/tests/lang/eval-okay-xml.exp.xml
@@ -31,9 +31,9 @@
     <attr name="f">
       <function>
         <attrspat>
-          <attr name="z" />
           <attr name="x" />
           <attr name="y" />
+          <attr name="z" />
         </attrspat>
       </function>
     </attr>