ENH: Deprecation for invalid fancy index sequences.

This raises a `DeprecationWarning` when a non-array fancy index sequence a) is not a sequence of booleans b) contains something not castable to npy_intp (i.e. a float). The way it gets there is a bit shaky, but it works. I'd like to solicit feedback on the best way to do this before merging.

What this does do, so far, is add tests, and also identify where the changes need to be made. A better implementation of `_verify_index` is definitely possible and probably preferable.