Reject derivable type classes with higher-rank methods

Trac #864 suggested a derivable type class with a higher-rank method.

In principle this is quite do-able, but in practice the mechanism works
by generating source code and then doing type inference.  To make this work
with higher-rank types would require impredicative polymorphism. And we
do have that, so it could probably be made to work by generating (source-level)
type annotations.  But it's real work, so I'm settling for generating a
decent error message rather than crashing.