Add a more memory-efficient RangeIndex-sort of thing to avoid large arange(N) indexes in some cases

[wesm]

[hayd]

Here's @jtratner's tree for this https://github.com/jtratner/pandas/tree/add-range-index (based off Wes').

(I keep struggling to find it. Perhaps I'll rebase and PR, would be interesting to experiment with this.)

[immerrr]

And there's likely some code that can be salvaged from BlockPlacement class I've added when refactoring block managers.