Add CasePathIterable and CasePathReflectable protocols

[stephencelis]

While the macro recently introduced iteration (#159) and "reflection" (#158), there's no way to abstract over it. Adding it directly to the CasePathable protocol would be potentially source breaking, so instead we can introduce some new protocols.

[stephencelis]

@mbrandonw I think this is ready to go. The main thing is if we're happy with the naming. CasePathIterable seems non-controversial, but the CasePathReflectable naming may be a little strange given that it's defined on the AllCasePaths type. One might assume/consider that the root enum is the "reflectable" type, and the AllCasePaths is just doing the reflecting...so is there a better name? CasePathReflecting maybe? Though I can't think of any protocols with an "-ing" suffix off the top of my head.