Allow creating an AHashMap without an `Eq` or `Hash` bound on the keys

[jyn514]

with_capacity_and_hasher requires K to implement Eq:

error[E0277]: the trait bound `K: Hash` is not satisfied
  --> compiler/rustc_data_structures/src/sso/map.rs:89:29
   |
89 |             SsoHashMap::Map(FxHashMap::with_capacity_and_hasher(cap, Default::default()))
   |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Hash` is not implemented for `K`
   |
   = note: required by `AHashMap::<K, V, S>::with_capacity_and_hasher`

Consider relaxing that bound, I think you shouldn't need it for constructing the hashmap, only for operations involving the elements (get/set/entry). Same request for various other functions, new(), capacity(), len(), etc.

Note the error is misleading here - FxHashMap is an alias for AHashMap that I renamed because it was simpler than changing a hundred imports.

Originally posted by @jyn514 in rust-lang/rust#77996 (comment)

[tkaitchuck]

Done 00d7e74

[jyn514]

Sorry, my request wasn't clear: I said 'Eq' but I meant 'neither Eq not Hash'. Both aren't necessary for these functions.

[tkaitchuck]

@jyn514 What are you doing with these hashmaps that don't contain any items?!

[jyn514]

Don't ask me, ask rustc 😆 Small set optimizations! https://doc.rust-lang.org/nightly/nightly-rustc/rustc_data_structures/sso/enum.SsoHashMap.html

[tkaitchuck]

Ok. Done 23af182
Released as 0.5.3