[PATCH 0 of 4] Implement basic sparse op diag and diag op sparse support.

[Jason Riedy]

Only *, /, \, +, and - are implemented.  Those are the ones that would be nicest to have, at
least for me.  Because the support is limited to those operations, I avoided serious macro
hackery and most of the auto-generation mechanism.  It's overkill here.

I'll poke at permutation matrices next.  I have code that works, but I need to clean it up
considerably.

I'd love to see this considered for 3.2.  Having eye (N) * A stay sparse would be great,
as would regularization_factor * eye (N) + A.  Code would be much more readable.  The only
possible downside would be if someone is relying on multiplication by eye () to make a
sparse matrix dense.

Jason