Add operation::svec() to access all s

I don't remember what I'm using this for, but it seems kind of
reasonable to me.
Add CATD instruction

This is designed to be exactly like the CAT instruction, but used only
during debugging.  I'm using this for the flo-mwe, which splits wide
operations into word-length operations.  This allows me to have a
pass-through so I can match VCD files.