Revive Cocoa support and add tests

I would like to see the Objective-C (Cocoa) support brought back. The suggestion to use Swift is not valid for all use cases for a few reasons:

1. Objective-C is not deprecated and is not going away.
2. Swift has to be written with Objective-C in mind to be backwards compatible. This severely limits the ability to use modern Swift. Plus, it requires bringing in the Swift runtime (several MB). It is nice to use all modern Swift features when using Swift and not have to be reminded of Objective-C backwards compatibility.
3. Objective-C library can be used with Swift with no extra cost, but not the other way around. Currently 0.12 already has good support by using modern Objective-C enums and could be expanded for better Swift support, for example adding nullability type annotations.
4. Swift cannot interact with C++, so if a project is mostly C++ that must interact with Cocoa APIs it must use Objective-C.

I am working on adding tests for the Cocoa code from the 0.12 version. My work in progress is here: https://github.com/kainjow/thrift/commits/kwojniak/cocoa-tests-wip