Merge metadata registrations

[ronaldoussoren]

For the support code for NS_UNAVAILABLE it would be nice to automatically merge metadata registrations for exactly the same classname/selectorname tuples.

Also allow setting "suggestion" to None to clear the suggestion field of the method signature (which would allow both setting and clearing that field when merging is implemented). One thing to investigate before implementing this: this might still be problematic when this is used to mark an "unavailable" selector as available in a subclass when that selector has non-trivial metadata in the parent class (such as completionhandler arguments).