internal lint: forbid HashMap and HashSet and suggest the FxHash* variants

[oli-obk]

They are much more performant and we don't need any collision prevention in clippy.

Once it works nicely we can upstream it to rustc, because they totally need that, too

[dwijnand]

I might have a crack at this. Is there any prior art I can emulate for this? Are there any more mentoring instructions you could add to this, please? The more the merrier as I'm a Rust novice, and this is my first venture into the world of Clippy.

[oli-obk]

The simplest solution (not quite correct, but I don't think we care) is to basically create a lint that only implements check_ident and if the argument is equal to HashMap or HashSet, we lint and suggest to use FxHashMap and FxHashSet.

Doing this by name obviously is not right, but anyone naming their things HashMap or HashSet should be angrily stared at anyway.

This lint is only for inside clippy and rustc, so the false positives aren't problematic

[oli-obk]

Basically this lint but with other names ;)

[mati865]

It may become relic of the past when rust-lang/rust#56241 gets merged.