Catch up with recent changes in the middle end that changed the
assumptions about constructor applications: it is not necessarily the
case any more that constructor applications are always saturated in
the output from CorePrep.

For a non-saturated constructor application there is always a curried
worker function that can be called instead.  This change updates the
byte code generator to use the curried worker when necessary.

Also: a couple of micro-optimisations/cleanups.