Objective-C property wrapping

[christiangnrd]

Adds what I need to wrap Objective-C Interfaces and Protocols and their properties. No methods yet.

I did my best to split this up in reasonable commits to make review easier and includes some tests.

[Gnimuc]

I never used Objective-C, does it share the same compilation environment/flags with C? I mean, do we need a new ObjectiveCContext <: AbstractContext dedicated to it?

[christiangnrd]

I don't know if we need a new Context, but it may be worth adding objectiveC to get_default_args and friends. I'll take a look in the next few days.

My goal with this is to get it to work for wrapping the Metal, MetalPerformanceShaders, etc. system frameworks while also having it be good enough to merge, but I may need your assistance with the environment, build arguments, etc as my only exposure to ObjectiveC has been with my quest to get these frameworks wrapped.

I'll take a look on my own at some point, but if you have the time (and desire) to take a look before I do, the second (much more hacky) half of this project lives in JuliaGPU/Metal.jl#526 (specifically res/wrap/wrap.jl)