Swift lang syntax to unite types

[esuvorov]

Unite swift lang syntax types with SwiftLangSyntax protocol
Make SwiftDocKey public

[jpsim]

@esuvorov thanks for the PR! Can you please explain the rationale behind these changes? I'm cautious when modifying the public API because it affects consumers of the library and restricts what kinds of changes can be made later.

[esuvorov]

@jpsim, hi!
I'm currently adding a couple ASTRule for SwiftLint which uses SourceKitten as dependency.

As i see SwiftLint framework use a lot of independent strings "key.everything" for AST parsing while in SourceKitten already exist SwiftDocKey. Of course i can add similar enum in local module, but as i think much better to use what we have instead of dublicate code.

Also in SwiftLint exist a couple methods which uses only for SyntaxKind, but they could be more usefully and flexible if we change SyntaxKind for SwiftLangSyntax abstraction.

So i made two main changes:

• SwiftLangSyntax protocol for unite source.lang.swift.* family for unification of StatementKind, SwiftDeclarationKind, SyntaxKind. (I'm not sure about the name of SwiftLangSyntax)
• and make SwiftDocKey public.

[jpsim]

Sorry for the long delay @esuvorov. Thanks for the PR, I'll accept this as a short term solution until SourceKitten can move away from untyped dictionaries towards typed declarations (#238).