Fix memory leak when breaking out of a for-in loop

Use an autoreleased NSMutableArray to keep the temporary objects alive rather
than a heap-allocated id[] so that the last block of objects will get
autoreleased if the iteration does not complete. This results in iteration
breaking if the user drains the autorelease pool while iterating over the
RLMArray, but that's no longer supported with ARC and was always a bad idea.
Switch RLMArrayLinkView's fast enumeration implementation to RLMArrayTableView's design

The previous implementation autoreleased the objects being looped over,
resulting in them staying alive unpleasantly long.
Update changelog
Verify that objects are released during fast enumeration and not just at the end
Use an autoreleased C array for the fast enumeration buffer

Seems to be about 10% faster than using NSMutableArray.
Merge pull request #722 from realm/tg-fast-enumerate

Fix memory leak when breaking out of a for-in loop