Add -Wrule-lhs-equalities warning

This commit adds a new warning, controlled by the warning flag,
-Wrule-lhs-equalities, which is emitted when the LHS of a RULE gives
rise to equality constraints that previous GHC versions would have
quantified over.

GHC instead discards such RULES, as GHC was never able to generate
a rule template that would ever fire; it's better to be explicit about
the fact that the RULE doesn't work.