Simon realised that the simple language composed of let bindings, bang
patterns and flat constructor patterns is enough to capture the
semantics of the source pattern language that are important for
pattern-match checking. Well, given that the Oracle is smart enough to
connect the dots in this less informationally dense form, which it is
now.

So we transform `translatePat` to return a list of `PmGrd`s relative to
an incoming match variable. `pmCheck` then trivially translates each of
the `PmGrd`s into constraints that the oracle understands.