When a pattern synonym is for an unlifted pattern, its "builder" would
naturally be a top-level unlifted binding, which isn't allowed.  So we
give it an extra Void# argument.

Our Plan A involved then making *two* Ids for these builders, with
some consequential fuss in the desugarer.  This was more pain than I
liked, so I've re-jigged it.

 * There is just one builder for a pattern synonym.

 * It may have an extra Void# arg, but this decision is signalled
   by the Bool in the psBuilder field.

   I did the same for the psMatcher field.

   Both Bools are serialised into interface files, so there is
   absolutely no doubt whether that extra Void# argument is required.

 * I renamed "wrapper" to "builder".  We have too may "wrappers"

 * In order to deal with typecchecking occurrences of P in expressions,
   I refactored the tcInferId code in TcExpr.

All of this allowed me to revert 5fe872
   "Apply compulsory unfoldings during desugaring, except for `seq` which is special."
which turned out to be a rather messy hack in DsBinds