* Improve the warning "M is imported but nothing from it is used"
  In particular, don't warn if some instances from it are imported.

  It's pretty much impossible to do the Right Thing always.
  A comment in Rename.lhs says
	-- NOTE: Consider
	--	      module This
	--		import M ()
	--
	--	 The import M() is not *necessarily* redundant, even if
	-- 	 we suck in no instance decls from M (e.g. it contains
	--	 no instance decls, or This contains no code).  It may be
	--	 that we import M solely to ensure that M's orphan instance
	--	 decls (or those in its imports) are visible to people who
	--	 import This.  Sigh.
	--	 There's really no good way to detect this, so the error message
	--	 in RnEnv.warnUnusedModules is weakened instead

* Minor comment changes to RnIfaces.lhs

* Use NameEnv instead of UFM in TcEnv (tidy up only)