Fixes ticket #5472

Better to specifically list the unsupported cases in code
than to have a catch all that panics. The later method hides
problems when new constructors are added such as the recent
additions to the supported Cmm prim ops that weren't ported
to the C backend since no one noticed.

hppa1, m68k

Profiling shows we're wasting a lot of time calling mkErrInfo and
tidying the context, work that is wasted if we're not tracing.

There is more to this compilation performance regression than this,
but this is one bug.

Also give a civilised error message when the data type to vectorised contains no data.

We were never really manipulating the EkCtxt after we had built it,
so it's simpler to just pass the final SDoc.

The previous names were not informative at all, and now we have
named kinds like Constraint and datatype promotion to kind, so
we might as well name these too.

I tried to update some comments to the new names, but certainly
many references to the old names remain.