Remove redundant nightly features
Removes a bunch of redundant/outdated nightly features. The first commit removes a `core_intrinsics` use for which a stable wrapper has been provided since. The second commit replaces the `const_generics` feature with `min_const_generics` which might get stabilized this year. The third commit is the result of a trial/error run of removing every single feature and then adding it back if compile failed. A bunch of unused features are the result that the third commit removes.
This introduces a no-op `Unhasher` for map keys that are already hash-
like, for example `Fingerprint` and its wrapper `DefPathHash`. For these
we can directly produce the `u64` hash for maps. The first use of this
is `def_path_hash_to_def_id: Option<UnhashMap<DefPathHash, DefId>>`.