[swift2objc] Support Protocols

[nikeokoronkwo]

From https://docs.swift.org/swift-book/documentation/the-swift-programming-language/protocols/#Protocol-Extensions:

A protocol defines a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. The protocol can then be adopted by a class, structure, or enumeration to provide an actual implementation of those requirements. Any type that satisfies the requirements of a protocol is said to conform to that protocol.

This pull request adds support for parsing protocols by achieving the following:

• Adding support for parsing basic protocols
• Adding parsing support for optional methods and properties
• Adding parsing support for conformed protocols (protocols that the given protocol inherits)
• Adding support for custom types in protocols via associatedTypes
• Adding support for methods and properties that act as default initialisations for protocols
• Writing tests that cover all given requirements

See #1828 for more information

[nikeokoronkwo]

Extension support (#1815) might need to be added to implement default initialisation functionality