----------------------------
	More jiggling in the renamer
	----------------------------

I was a little hasty before.  (Thanks Sigbjorn for finding
this.)  This commit tidies up the handling of AvailEnvs.
Principally:

  * filterImports now deals completely with hiding
    (before it handed off part of the job to mkGlobalRdrEnv)

  * The AvailEnv in an ExportAvails does not have class ops and
    data constructors in its domain.  This makes plusExportAvails
    more efficient, but the main thing is that it collects things
    up right.  (Previously, if we had
	import M( C )
	import M( op )
    then we got an AvailEnv which had C |-> AvailTC C [C]
    (no 'op').

  * In Rename, we do need a "filled-out" version of the overall
    AvailEnv, full_avail_env, which we construct on the spot in 'rename'.