Hurrah.  At last we can write

   class (F a ~ b) => C a b where { ... }

This fruit of the fact that equalities are now values,
and all evidence is handled uniformly.

The main tricky point is that when translating to Core
an evidence variable 'v' is represented either as
  either   Var v
  or       Coercion (CoVar v)
depending on whether or not v is an equality.  This leads
to a few annoying calls to 'varToCoreExpr'.