Rollup merge of #154634 - nnethercote:hcx, r=petrochenkov

Use `Hcx`/`hcx` consistently for `StableHashingContext`.

The `HashStable` and `ToStableHashKey` traits both have a type parameter that is sometimes called `CTX` and sometimes called `HCX`. (In practice this type parameter is always instantiated as `StableHashingContext`.) Similarly, variables with these types are sometimes called `ctx` and sometimes called `hcx`. This inconsistency has bugged me for some time.

The `HCX`/`hcx` form is more informative (the `H`/`h` indicates what type of context it is) and it matches other cases like `tcx`, `dcx`, `icx`.

Also, RFC 430 says that type parameters should have names that are "concise UpperCamelCase, usually single uppercase letter: T". In this case `H` feels insufficient, and `Hcx` feels better.

Therefore, this commit changes the code to use `Hcx`/`hcx` everywhere.

r? @petrochenkov