mat: add striding to DiagDense and capacity to extract diags from other matrices

Created by: kortschak

Please take a look.

I'm not really all that happy with the approach in DiagOf because of the mixing of view and copy, but the alternatives that I can see are less attractive; have a collection of methods for each raw type, either on DiagDense or (less likely) the raw types, or alternatively panicking when a non-raw type is handed in.

Fixes #609 (closed). Updates #213 (closed).