Consider a RULE like
	forall arr. splitD (joinD arr) = arr

Until now, this rule would not match code of form
	splitD (let { d = ... } in joinD (...d...))
because the 'let' got in the way.

This patch makes the rule-matcher robust to lets.  See comments with
the Let case of Rules.match.

This improvement is highly desirable in the fusion rules for NDP
stuff that Roman is working on, where we are doing fusion of *overloaded*
functions (which may look lazy).  The let expression that Roman tripped
up on was a dictioary binding.