[swift2objc] Support Type Constraints

[nikeokoronkwo]

Type Constraints are a Swift feature where the type in a generic function or class can be restricted based on whether it implements either a given protocol or a given class:

func myFunc<T: Equatable>(param: T) {
  // function body
}

Dart has a similar feature where a generic is constrained to extend a given class.
This should be relatively easy to parse, as the symbol graph provides constraints information concerning this

[nikeokoronkwo]

For the sake of completeness, this issue will also include support for type constraints for extensions, which is enforced in swift via the where clause.

extension Collection where Element: SomeProtocol {
    func someFunc() -> Bool where Element: Numeric {
        // function body
    }
}

[nikeokoronkwo]

Noticed that type generics in general haven't been supported in the parsing frontend, so will work on both together.