[WIP] Split RLMArray into RLMArray + RLMResults

Splits RLMArray into separate apis.

The relevant apis should now be updated along with inline docs. All osx tests currently pass. We will still need to update the rest of the docs on the website separately.

Next step is to fix swift and then examples.
@tgoyne any ideas on a not horrible hack we can do here? The best option I'm thinking at this point is to support a NULL _backingQuery for RLMResults, in which case we will return a count of 0. This would require an extra conditional for many RLMResults methods but is the easiest non-hacky solution I can think of. Another option would be a subclass of RLMResults which is always empty.