Change the renamer namesuppy for instance decls, so that
it is indexed by the class/tycon *string* rather than the
class/tycon pair. That way (C,TT) and (CT,T) both give
the string "CTT", and hence give a different unique.

An alternative would have been to use "C/TT" and "CT/T"
respectively, but that would mean obscure errors while everyone
remembered to recompile everything.  So this seems more direct.

Julian reported this bug.