Add approximately 25mL of Safety to NNCleanupProxy

Method signatures can't be fetched when the target object is dead, so
they must be cached ahead of time.
Never use exceptions for successful control paths. Even in tests.

And just yesterday I was asking "how can this bite me?" Didn't even
take a whole day.
Be more fatal when selectors are not predeclared.
Private docs
NNMutableWeakSet is finished, with enumerator support.
Whitespace
Not gonna worry about it.
Property should be readonly.