Cursor / Selection API changes in Editor

Here are my suggestions for editing the cursor and selection API. My main goal is consistency and symmetry. I'll pepper with inline comments explaining my decisions.
Do we do `getCursor(index)`, `getCursorAtIndex(index)` or `cursorAtIndex(index)`?

Maybe the objc form `cursorAtIndex(index)` is better for things with `for` in them: `decorationsForScreenRowRange`, etc. Just want to have a consistent way to go about these kinds of things.