Switch from a string core to a 128-bit integral core. This should make
Fingerprints much cheaper to copy around and sets us up for a future
where we can provide alternative implementations of the ambient hashing
algorithm.
rdar://72313506
A fingerprint is a stable hash of a particular piece of compiler data. This formalizes the stable notion of identity that the dependency trackers use for type body fingerprints in iterable decl contexts and the file-level interface hash
