Ability to define classes using native syntax

[maleadt]

Was removed as part of #11 (due to bitrot), so if anybody is interested in bringing this back, look at https://github.com/JuliaInterop/ObjectiveC.jl/tree/22118319da1fb7601d2a3ecefb671ffbb5e57012

[maleadt]

-@class

• uses old type definition syntax
• also needs syntax to specify which protocols are conformed too -- this is probably the most import use case for Julia users (e.g. define delegates)
• need a way to call methods in super (objc_sendSuper) -- needed in init usually
• ivars? if we only care about protocol support, we can elide.

Originally posted by @habemus-papadum in #11 (comment)

[habemus-papadum]

• (another nice to have) Objective-C Categories (add methods to existing classes -- a useful pattern in objective-c land)