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	Performance tuning things
	-------------------------

I did some nofib tests, and fixed a number of performance problems.

1.  Things were getting floated to top level, and that prevented
some useful fusion happening.
	y = build g
	x = foldr k z y

Fixed by arranging that we only get really keen on floating to top
level in the second run of the let-float-out pass.


2.  Some fettling up on the let-floater itself.  It had some parameters
that weren't even being used!  And it was stupidly floating things out
of a one-shot lambda, and the float-in pass didn't float them back in.
I think I fixed both of these problems.


3.  The eta-reducer was not eta-reducing (/\a -> g a) to g.  In general
it has to be a bit careful because "seq" means that (\x -> g x) is
not in general the same as g ---- but it *is* the same for a type lambda.

This turned out to be important in rule matching, where the foldr/build
rule was not firing because the LHS of t...