[SourceKit] Add SwiftLang API to parse into ByteTree format
Redesigns the `SwiftLang.parse` API to accommodate a format parameter which can be used to request a ByteTree-serialized syntax tree instead of JSON-serialized. This gives us a convenient way to use the fastest currently available combination of tools for parsing a file into a syntax tree.
The new API only accepts paths, not source buffers, and returns JSON as `Data` instead of `String`, which are both better directions in the long run. The format type is extensible so that it can easily be extended to support "direct" parsing into a SyntaxSourceFile.
Deprecated wrapper methods with the old names and signatures are still available for any existing clients.
