feat: Add a compile-time hash map type

[jfecher]

Description

Problem*

Resolves #5479

Summary*

Mostly copied over from our existing hash map with a few changes.

• Uses slices and grows automatically
• Uses break

Additional Context

Like other comptime code, I'm avoiding documenting this until comptime as a whole is stable

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[jfecher]

Wondering if this should be renamed since there's nothing inherently comptime about this HashMap.
Maybe we should brand it as an UnconstrainedHashMap instead? UHashMap?

[michaeljklein]

LGTM

R.e. naming:

• UHashMap makes sense to me.
• Since it's in the meta module already, what about simply naming it HashMap and differentiating it by path as needed?

[jfecher]

Since it's in the meta module already, what about simply naming it HashMap and differentiating it by path as needed?

I like the two levels since most will import the name directly into scope anyway. When this happens it'd be confusing reading HashMap in a codebase and not knowing if it was the constrained or unconstrained kind. I also think I'm going to move it out of the meta module since it no longer has to do with metaprogramming. I'll probably move it to std::collections::umap for unconstrained map.