std: make ArrayHashMap eql function accept an additional param

[andrewrk]

which is the index of the key that already exists in the hash map.

This enables the use case of using AutoArrayHashMap(void, void) which
may seem surprising at first, but is actually pretty handy!
This commit includes a proof-of-concept of how I want to use it, with a
new InternArena abstraction for stage2 that provides a compact way to
store values (and types) in an "internment arena", thus making types
stored exactly once (per arena), representable with a single u32 as a
reference to a type within an InternArena, and comparable with a
simple u32 integer comparison. If both types are in the same
InternArena, you can check if they are equal by seeing if their index is
the same.

What's neat about AutoArrayHashMap(void, void) is that it allows us to
look up the indexes by key, without actually storing the keys.
Instead, keys are treated as ephemeral values that are constructed as
needed.

As a result, we have an extremely efficient encoding of types and
values, represented only by three arrays, which has no pointers, and can
therefore be serialized and deserialized by a single writev/readv call.
The map field is denormalized data and can be computed from the other
two fields.

This is in contrast to our current Type/Value system which makes
extensive use of pointers.

The test at the bottom of InternArena.zig passes in this commit.

[andrewrk]

I added the "breaking" label but note that it's only breaking for the "adapted" variants of the ArrayHashMap API. Regular usage with AutoArrayHashMap is unaffected.