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.
It doesn't seem very useful to me, but I included everything from `NSSortDescriptor` that's applicable. I don't know how `RLMSortDescriptor` being immutable is relevant.