refactor(3p/nix/libexpr): state->allocBindings -> Bindings::NewGC

EvalState::allocBindings had little to do with Bindings, other than
returning them, and didn't belong in that class.

third_party/nix/src/libexpr/attr-set.hh

@@ -33,6 +33,10 @@ class Bindings {
  public:
   typedef absl::btree_map<Symbol, Attr>::iterator iterator;
 
+  // Allocate a new attribute set that is visible to the garbage
+  // collector.
+  static Bindings* NewGC();
+
   // Return the number of contained elements.
   size_t size();