Deprecations [HscTypes, MkIface, Rename, RnEnv, RnIfaces]
~~~~~~~~~~~~
* Arrange that a change in deprecations is treated as a 
  hi-file difference.

* Warn about deprecations at the usage site.  This entailed
  changing HscTypes.GlobalRdrEnv to include deprecations.
  While I was at it, I changed the range of GlobalRdrEnv 
  to a data type, GlobalRdrElt, instead a of a pair.