Implicit parameters should not be allowed in class and instance declarations

Trac #8912 pointed out that GHC 7.4 and 7.6 have omitted this test, although
7.2 and earlier had it.  This patch puts the test back in, and refactors a
little.

(cherry picked from commit a8b7b28c)

Conflicts:
	testsuite/tests/typecheck/should_fail/all.T