Improve IfaceSyn a bit further

This patch has three main bits:

* The most substantial change is that IfaceConDecl no longer
  records its universal type variables, because they are
  always the same as those of the parent TyCon.  A bit less
  fuss and clutter.

* Add a synonym for IfTopBndr = OccName, and explain why it's an
  OccName not a FastString

* Make the ifMinDef field be a (BooleanFormula IfLclName) rather
  than (BooleanFormula OccName).  These really are occurrences (not
  binders), and should be treated like other occurences.

The first and third change the format of interface files, so
you'll need to recompile.