Tune up the reporting of unused imports

		Merge to STABLE
	(I think the earlier change made it across)
	(PS: the commit also does some trimming of
	redundant imports.  If they don't merge, just
	discard them.)

My earlier fixes to the reporting of unused imports still missed
some obscure cases, some of which are now fixed by this commit.
I had to make the import-provenance data type yet richer, but in
fact it has more sharing now, so it may be cheaper on space.

There's still one infelicity. Consider
		import M( x )
		imoprt N( x )
where the same underlying 'x' is involved in both cases.  Currently we
don't report a redundant import, because dropping either import would
change the qualified names in scope (M.x, N.x). But if the qualified
names aren't used, the import is indeed redundant. Sadly we don't know
that, because we only know what Names are used.  Left for the future!
There's a comment in RnNames.warnDuplicateImports

This commit also trims quite a few redundant imports disovered
by the new setup.