Avoid rehashing Fingerprint as a map key

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>>`.

compiler/rustc_data_structures/src/lib.rs

@@ -103,6 +103,7 @@ pub use atomic_ref::AtomicRef;
 pub mod frozen;
 pub mod tagged_ptr;
 pub mod temp_dir;
+pub mod unhash;
 
 pub struct OnDrop<F: Fn()>(pub F);