ENH: enable mul, div on Index by dispatching to Series
One of the last remaining inconsistencies between Index and Series arithmetic is that division and multiplication is entirely disabled for object-dtype Index, while for Series we operate pointwise.  This PR makes Index behave the same way, and simplifies the code by using existing Series-op-wrapping code to do so.

- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I'm pretty sure there is at least one issue for this, will track it down.