Do something with NS_UNAVAILABLE annotations in header files

[ronaldoussoren]

Original report by Ronald Oussoren (Bitbucket: ronaldoussoren, GitHub: ronaldoussoren).

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Apple's header files have annotations on some methods to indicate that you shouldn't call methods on a subclass (mostly about -init not being available in some classes).

Add metadata about that and emit a warning when those methods are called anyway (the warnings machinery can be used to convert those into exceptions).

[ronaldoussoren]

This is information that can be extracted from header files, and an in-development version of objective.metadata does just that.

Keeping this issue open until that work is actually used.