Hs-boot files can contain instance declarations, but Duncan used this feature
for the first time today, and it didn't quite work.  I'm not sure whether
it worked before; anyway it does now.

The point is that the hs-boot file advertises an instance for, say, Num Int,
with the arbitrary name $fx1.  The main module declares Num Int, and gives
it the name, say, $f3.  So we need to generate a declaration
	$fx1 = $f3
to make it all line up.  And (this is the bit that was wrong) we need
to make that new binding visible to imported unfoldings that mention
$fx1.