feat(3p/nix): Add vector-backed impl for Bindings r/1284

Add an alternative impl of the now-abstract Bindings base class that is
backed by a std::vector, somewhat similar but stylistically a little
superior to the array-backed implementation in upstream nix. The
underlying iterator type in BindingsIterator is now backed by a
std::variant that we std::visit an overload over in order to implement
the various bits of the iterator interface.

Paired-With: Luke Granger-Brown <git@lukegb.com>
Paired-With: Vincent Ambo <mail@tazj.in>
Paired-With: Perry Lorier <isomer@tvl.fyi>
Change-Id: I7fbd1f4d5c449e2f9b82102a701b0bacd5e80672
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1123
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

0 files changed, 0 insertions, 0 deletions