-------------------------------------
	Improve the "unused binding" warnings
	-------------------------------------

We've had a succession of hacks for reporting warnings for
unused bindings.  Consider

	module M( f ) where

 	f x = x

	g x = g x + h x
	h x = x

Here, g mentions itself and h, but is not itself mentioned. So
really both g and h are dead code.  We've been getting this wrong
for ages, and every hack so far has failed on some simple programs.

This commit does a much better job.  The renamer applied to a bunch
of bindings returns a NameSet.DefUses, which is a dependency-ordered
lists of def/use pairs.  It's documented in NameSet.
Given this, we can work out precisely what is not used, in a nice
tidy way.

It's less convenient in the case of type and class declarations, because
the strongly-connected-component analysis can span module boundaries.
So things are pretty much as they were for these.


As usual, there was a lot of chuffing around tidying things up.
I havn't tested it at all thoroughly yet.

Various unrelated import-decl-pruning has been done too.