Implement a form of CAT

This only works when there's a single word that's being cat'd onto a
potentially multi-word operation, but that should be the common case.
Generate CATD widths correctly

I was generating nodes with the wrong CATD widths.  I have a width bug
and am therefor attempting to add some width sanity checking to libflo
graphs, which "discovered" (though I knew about it, I was just lazy)
this bug.
Fix memory constant generation

I can now generate constants of fixed widths, which will probably
prove useful for the future.  It's also necessary to correctly
generate a constant inside the memory splitting stuff.