This was a serious bug, exposed by Trac #9175.  The matcher and wrapper
must be LocalIds, like record selectors and dictionary functions, for
the reasons now documented in Note [Exported LocalIds] in Id.lhs

In fixing this I found
 - PatSyn should have an Id inside it (apart from the wrapper and matcher)
   It should be a Name.  Hence psId --> psName, with knock-on consequences

 - Tidying of PatSyns in TidyPgm was wrong

 - The keep-alive set in Desugar.deSugar (now) doesn't need pattern synonyms
   in it

I also cleaned up the interface to PatSyn a little, so there's a tiny knock-on
effect in Haddock; hence the haddock submodule update.

It's very hard to make a test for this bug, so I haven't.