Deduplicate Proto associated type in {From,TryFrom,Into}Proto

[benluelo]

Currently, {From,TryFrom,Into}Proto each define their own associated type Proto, which is (silently) expected to be the same for Into and [Try]From. This is a relic of early prototyping that made it easier to rapidly iterate (as implementing Into didn't require implementing [Try]From, etc).

• Define one trait Proto that defines an associated type Proto
  • Perhaps name the associated type Raw?
• Add Proto as a supertrait of {From,TryFrom,Into}Proto
  • {From,TryFrom,Into}Proto will become convenience traits over {From,TryFrom,Into}<<Self as Proto>::Proto>

NOTE: Due to the never type still being unstable (:pensive:), FromProto and TryFromProto will remain as a way to differentiate fallible and infallible conversion (as opposed to putting everything in Proto). If Result::into_ok was stable, that could be used instead as a hypothetical Proto::Error type, but unfortunately it seems to be quite far away from stabilization. We could define an extension trait, but that results in rust-lang/rust#48919, which is annoying (and I'm not sure what else to call the method).