This commit implements the stabilization of the custom hasher support intended
for 1.7 but left out due to some last-minute questions that needed some
decisions. A summary of the actions done in this PR are:

Stable

* `std::hash::BuildHasher`
* `BuildHasher::Hasher`
* `BuildHasher::build_hasher`
* `std::hash::BuildHasherDefault`
* `HashMap::with_hasher`
* `HashMap::with_capacity_and_hasher`
* `HashSet::with_hasher`
* `HashSet::with_capacity_and_hasher`
* `std::collections::hash_map::RandomState`
* `RandomState::new`

Deprecated

* `std::collections::hash_state`
* `std::collections::hash_state::HashState` - this trait was also moved into
  `std::hash` with a reexport here to ensure that we can have a blanket impl to
  prevent immediate breakage on nightly. Note that this is unstable in both
  location.
* `HashMap::with_hash_state` - renamed
* `HashMap::with_capacity_and_hash_state` - renamed
* `HashSet::with_hash_state` - renamed
* `HashSet::with_capacity_and_hash_state` - renamed

Closes #27713