refactor(3p/nix): Revert VectorBindings implementation

This reverts parts of the CLs splitting the backing implementation for
Bindings and moves back to only the BTreeMap-backed implementation.

Our evaluation has indicated that the Vector-backed implementation
does not match the performance of the plain array used upstream, and
in my view the complexity introduced by it is not worth the relatively
small (single-digit percentage) performance increase with a
pivot-point close to the number of attributes yielded by
stdenv.mkDerivation.

Going forward we will trial implementations of attribute sets backed
by HAMTs, and investigate other mechanisms of speeding up the language.

Some changes from the previous CLs are retained, for example the
removal of insert_or_assign and the passing of capacity.

Change-Id: I6eb4b075b453949583360755055c21a29d7ff642
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1172
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

third_party/nix/tests/lang.sh

@@ -1,5 +1,3 @@
-source common.sh
-
 export TEST_VAR=foo # for eval-okay-getenv.nix
 
 nix-instantiate --eval -E 'builtins.trace "Hello" 123' 2>&1 | grep -q Hello