%clear should also delete _NN references and Out[NN] ones

We need a few things regarding variable/namespace clearing:

- A 'hard reset' that really nukes _NN, Out[NN], _, __, ___ and all other references we create behind the user's back.

- Visible name reset: what %reset does today.

- Integration of %reset and %clear in a nice, unified interface for cleaning various parts of the things we cache. Users shouldn't have to know about these two.

I'm making this high (but not critical) importance because it is a problem that keeps coming back on the mailing lists.