bulk-allocate Value instances in the evaluator

calling GC_malloc for each value is significantly more expensive than
allocating a bunch of values at once with GC_malloc_many. "a bunch" here
is a GC block size, ie 16KiB or less.

this gives a 1.5% performance boost when evaluating our nixos system.

tested with

nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'

 # on master

  Time (mean ± σ):      3.335 s ±  0.007 s    [User: 2.774 s, System: 0.293 s]
  Range (min … max):    3.315 s …  3.347 s    50 runs

 # with this change

  Time (mean ± σ):      3.288 s ±  0.006 s    [User: 2.728 s, System: 0.292 s]
  Range (min … max):    3.274 s …  3.307 s    50 runs

src/libexpr/eval.hh

@@ -133,6 +133,9 @@ private:
     /* Cache used by prim_match(). */
     std::shared_ptr<RegexCache> regexCache;
 
+    /* Allocation cache for GC'd Value objects. */
+    void * valueAllocCache = nullptr;
+
 public:
 
     EvalState(