-----------------------------
	Fix a too-vigorous export bug
	-----------------------------

	MERGE TO STABLE

	[this is the Ian Lynagh -O2 bug]


CoreTidy didn't filter the rules that it exports, so it
exported some that mentioned Ids on the *left* hand side
that are not exported.  So an importing module fell over.

The fix is simple: filter the exposed rules.   On the way
I tidied up CoreFVs a little.