[swift2objc] Support Parsing Generic Types and Type Constraints

[nikeokoronkwo]

Generic Types in Swift are similar to generics in Dart, and are used for similar purposes in structs, classes, typealiases and functions.

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 pull request adds support for not only parsing type generics but also for parsing type constraints by

• Adding support for parsing type generics in Swift
• Being able to easily access generics used in compound declarations from the methods, properties and nested declarations in it
• Including fields for the constraints applied on the given generic type
• Adding support for parsing such constraints from functions
• Adding support for parsing such constraints from compound declarations
• Adding support for getting type constraints used in Swift Extensions

More information can be found here: #1814

[nikeokoronkwo]

At the moment, Type Constraint tests do not succeed completely because support for protocols have not been implemented, so either of #1828 or #1827 have to be implemented to make successful tests for these