[QuickSpec] Track the current example within `imp_implementationWithBlock`
[QuickSpec] Track the current example within `imp_implementationWithBlock`

This eliminates Objective-C's associated objects usage and overriding `-setInvocation:`.

phatblat/Quick,DanielAsher/Quick,phatblat/Quick,phatblat/Quick,phatblat/Quick,Quick/Quick,Quick/Quick,Quick/Quick,DanielAsher/Quick,Quick/Quick