Add builtin function "partition"

The implementation of "partition" in Nixpkgs is O(n^2) (because of the
use of ++), and for some reason was causing stack overflows in
multi-threaded evaluation (not sure why).

This reduces "nix-env -qa --drv-path" runtime by 0.197s and memory
usage by 298 MiB (in non-Boehm mode).

src/libexpr/eval.hh

@@ -71,7 +71,8 @@ public:
 
     const Symbol sWith, sOutPath, sDrvPath, sType, sMeta, sName, sValue,
         sSystem, sOverrides, sOutputs, sOutputName, sIgnoreNulls,
-        sFile, sLine, sColumn, sFunctor, sToString;
+        sFile, sLine, sColumn, sFunctor, sToString,
+        sRight, sWrong;
     Symbol sDerivationNix;
 
     /* If set, force copying files to the Nix store even if they