Improved RULE lhs typechecking; less dictionary sharing

Improved RULE lhs typechecking; less dictionary sharing

See long comment with Simplify.tcSimplifyRuleLhs.

Here's the key example:

  RULE "g"  forall x y z. g (x == y) (y == z) = ...

Here, the two dictionaries are *identical*, but we do NOT WANT to
generate the rule

RULE	forall x::a, y::a, z::a, d1::Eq a
	  f ((==) d1 x y) ((>) d1 y z) = ...

Instead we want

RULE	forall x::a, y::a, z::a, d1::Eq a, d2:Eq a
	  f ((==) d1 x y) ((>) d2 y z) = ...