rustc encodes a lot of hashes. If hashes are stored as u64 or u128, a derive(Encodable)
will apply varint encoding to the hashes, which is less efficient than directly encoding the 8
or 16 bytes of the hash. And if that hash depends on the StableCrateHash (which most in rustc
do), the varint encoding will make the number of bytes encoded fluctuate between compiler
versions.
The types in this module represent 64-bit or 128-bit hashes produced by a StableHasher.
Hash64 and Hash128 expose some utility functions to encourage users to not extract the inner
hash value as an integer type and accidentally apply varint encoding to it.
In contrast with Fingerprint, users of these types cannot and should not attempt to construct
and decompose these types into constituent pieces. The point of these types is only to
connect the fact that they can only be produced by a StableHasher to their
Encode/Decode impls.