This way we only mix the fieldNumber once for these fields instead
of once per entry. The extra data into the hash doesn't really
buy us anything, and just slows down hash performance.

Conditional conformance on the containers now make them Hashable if they value
type is hashable. Use this instead of manually doing the hashing of the values
in the containers to get any standard library improvements.

https://swift.org/blog/conditional-conformance/ documents this as Swift 4.1,
but there seem to have been issues, as it fails until Swift 4.2.