A bunch of small changes in the way that usage information
is generated to stuff into interface files.  I'm not certain
anything was really wrong before, but it's tidier now, and
there are slightly fewer dependencies generated.  Main
differences are in RnIfaces.getImportVersions

I also made the check for orphan rules a bit cleverer
(Rename.isOrphanDecl) so that we get fewer spurious orphan modules.

Simon