Without -O, we previously tried to make interface files smaller
by not including the data constructors of data types.  But
there are a lot of exceptions, notably when Template Haskell is
involved or, more recently, DataKinds.

However Trac #7445 shows that even without TemplateHaskell, using
the Data class and invoking Language.Haskell.TH.Quote.dataToExpQ
is enough to require us to expose the data constructors.

So I've given up on this "optimisation" -- it's probably not
important anyway.  Now I'm simply not attempting to trim off
the data constructors.  The gain in simplicity is worth the
modest cost in interface file growth, which is limited to the
bits reqd to describe those data constructors.