[ios, macos] Raise NSException if layer or source added more than once

When MGLSource and MGLLayer instances are added to the style, they lose
ownership of their std::unique_ptr<T> after it is moved to the mbgl
level. Subsequent attempts to add such instances result in a C++
exception.

This adds logic in the Darwin platform to raise a NSException if the
source and layer addToMapView methods are called more than once and the
pointer is invalid. In addition, the documentation in MGLStyle for
addSource: and addLayer: has been update to warn developers to avoid
adding the same instance twice and, for that matter, instances with the
same identifier.

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