refactor(3p/nix/libexpr): Back Nix lists with std::vector

This change does away with the previous special-casing of lists of
certain element sizes, and the use of raw C-style arrays.

Lists are now backed by a std::vector of nix::Value*, which uses the
traceable GC allocator.

This change is unfortunately quite noisy because the accessor methods
were updated/removed accordingly, so all callsites of Nix-related
lists have changed.

For some operations in primops.cc where keeping the previous code
structure would have been more difficult with a "proper" vector, the
implementation has been replaced with std::vector methods. For
example, list concatenation now uses appropriate range inserts.

Anecdotally the performance of this is about equal, to even slightly
better, than the previous implementation.

All language tests pass and the depot paths I've used for testing
still evaluate.

Change-Id: Ib5eca6c0207429cb323a330c838c3a2200b2c693
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1266
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>
Reviewed-by: Kane York <rikingcoding@gmail.com>
Reviewed-by: glittershark <grfn@gws.fyi>

third_party/nix/src/libexpr/eval.hh

@@ -260,12 +260,19 @@ class EvalState {
 
   Value* allocAttr(Value& vAttrs, const Symbol& name);
 
-  void mkList(Value& v, size_t size);
+  // Create a list value from the specified vector.
+  void mkList(Value& v, NixList* list);
+
+  // Create a list value, allocating as many elements as specified in
+  // size. This is used for the many cases in this codebase where
+  // assignment happens into the preallocated list.
+  void mkList(Value& v, size_t size = 0);
+
   void mkAttrs(Value& v, size_t capacity);
   void mkThunk_(Value& v, Expr* expr);
   void mkPos(Value& v, Pos* pos);
 
-  void concatLists(Value& v, size_t nrLists, Value** lists, const Pos& pos);
+  void concatLists(Value& v, const NixList& lists, const Pos& pos);
 
   /* Print statistics. */
   void printStats();