Make recursion and RULES interact better

See Trac #683

This patch improves the interaction of recursion and RULES; at least I
hope it does.   The problem was that a RULE was being treated uniformly like
an "extra RHS". This worked badly when you have a non-recursive definition
that is made recursive only by RULE.

This patch maeks the occurrence analyser know whether a binder is referred to
only from RULES (the RulesOnly constructor in OccInfo).  Then we can ignore
such edges when deciding on the order of bindings in a letrec, and when
setting the LoopBreaker flag.

The remaining potential problem is this:
	rec{ f = ...g...
	   ; g = ...f...
	     RULE g True = ...
	   }

The RULE for g may not be visible in f's rhs.  This is fixable, but not
today.