Move Hashable, Comparable conformance to DeclarationType.

[paulyoung]

As discussed in https://github.com/jpsim/SourceKitten/pull/112/files#r46729258.

This also adds AnyDeclarationType for reasons explained in https://github.com/jpsim/SourceKitten/pull/112/files#r46729614

[paulyoung]

cc @jpsim

[jpsim]

I'm wondering what's preferable: having dedicated SwiftDeclaration and ObjCDeclaration structs, or a single Declaration that exposes the joined set of members from both languages, asserting at runtime if trying to get say the ACL of an Objective-C declaration. Sorry for the back and forth on this, it just seems like having that represented in the type system might be adding more complexity when it's not necessary. Thoughts?

[paulyoung]

No worries. Sometimes you've got to see the complexity of something before you can simplify it.

I'm not doing much with Objective-C these days and try to leverage Swift's type system as much as possible, but I don't have a real preference here provided that the original issue I raised is addressed: jpsim#87 (comment)

[jpsim]

Yeah, I hear you, and totally appreciate your work on this. I just want to make sure we're on the right track!

[yoiang]

Hey y'all, any progress regarding this, #2 and related? Any way I can help?