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.
I can add a comment to the code. I wish this weren't needed, and thinking about it a bit more I'm not sure it really is. The only time it is really needed is for read-only realms where you are trying to access an object type which was not present when the realm file was created. In these cases I think it would be ok to throw an exception rather than returning an empty table. Thoughts @tgoyne  ?