Refactor to combine two eqExpr functions

I'd forgotten that Rules.lhs already has an eqExpr function.  This
patch combines Rules.eqExpr with the (recent) CoreUtils.eqExpr.

I also did a little refactoring by defining CoreSyn.expandUnfolding_maybe
(see Note [Expanding variables] in Rules.lhs), and using it in
     a) CoreUnfold.exprIsConApp_maybe
     b) Rule matching