Allow explicit block-passing to on_entry/on_exit

[buffpojken]

Due to Rubymotion adding support WeakRef, and weakly refed callables, not being able to mark a callback as weak introduces subtle bugs and memory leaks if user references for example a delegate-object within a block.

This allows a block to be marked as weak! and then passed as an argument.

[opyh]

I managed to overlook this pull request, sorry for that! Very good idea, it's just missing a spec…