Fix two related bugs in u_tys

When we normalise a type family application we must recursively call
uTys, *not* 'go', because the latter loop is only there to look
through type synonyms.  This bug made the type checker generate
ill-typed coercions, which were rejected by Core Lint.

A related bug only affects the size of coercions.  If faced with
  (m a) ~ (F b c)
where F has arity 1, we want to decompose to
   m ~ F Int,  a ~ c
rather than deferring.  The application decomposition was being
tried last, so we were missing this opportunity.

Thanks to Roman for an example that showed all this up.