Audit headers for nullability

[1ec5]

All the headers have been audited for nullability. This is a breaking change in the sense that Swift code written against Mapbox GL will likely fail to compile. However, Xcode will offer fix-its for every error that comes up, and the resulting code will be much less stressful (less shouting!) and more idiomatic.

Fixes #1054.

/cc @incanus

[1ec5]

@incanus @bleege @friedbunny, just a heads-up that this PR brackets @interfaces in NS_ASSUME_NONNULL_BEGIN/NS_ASSUME_NONNULL_END. The NS_ASSUME_NONNULL_* macros keep the headers from getting incredibly messy. So once this PR is merged, if you add any public methods that receive or return a pointer type, remember to add the nullable annotation if the value may be nil for any reason.

[1ec5]

This needs to be on beta 2. Apple finally audited MapKit for nullability with Xcode 7 and the iOS 9 SDK, and we need to be compatible.

[1ec5]

Looks like I’ll need to also bring over much of 54e2bb8 to satisfy a warning about invoking a non-designated initializer in MGLUserLocationAnnotationView.

[incanus]

Apple finally audited MapKit for nullability with Xcode 7 and the iOS 9 SDK, and we need to be compatible.

Is this mostly from a "so it still looks like Apple" POV? Or something more drastic as to why we should hit this on the sooner side?

[1ec5]

Mostly from a “so it still looks like Apple” POV. But also so we can say we also audited Mapbox GL for nullability, just as Apple announced this week, and because this work has no stability implications.

[jfirebaugh]

@1ec5 If you are happy with this please rebase and merge it. 🚢