feat(3p/nix): Statically pass bindings capacity where possible

To aid in making the decision of where to (currently just statically)
use a vector or btree as the backing implementation, add an extra
constructor argument to Bindings::NewGC for a capacity, and use
a (currently hardcoded at 32, for no good reason other than it felt like
a reasonable number) pivot to switch between our possible backing
implementations. Then, update all the call sites where it feels
reasonable that we know the capacity statically to *pass* that capacity
to the constructor.

Paired-With: Luke Granger-Brown <git@lukegb.com>
Paired-With: Vincent Ambo <mail@tazj.in>
Paired-With: Perry Lorier <isomer@tvl.fyi>
Change-Id: I1858c161301a1cd0e83aeeb9a58839378869e71d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1124
Tested-by: BuildkiteCI
Reviewed-by: lukegb <lukegb@tvl.fyi>
Reviewed-by: isomer <isomer@tvl.fyi>

third_party/nix/src/libexpr/common-eval-args.cc

@@ -33,7 +33,7 @@ MixEvalArgs::MixEvalArgs() {
 }
 
 Bindings* MixEvalArgs::getAutoArgs(EvalState& state) {
-  Bindings* res = Bindings::NewGC();
+  Bindings* res = Bindings::NewGC(autoArgs.size());
   for (auto& i : autoArgs) {
     Value* v = state.allocValue();
     if (i.second[0] == 'E') {