Cleanups:

- Move the collect* functions from HsSyn into HsUtils.  Check that we
  have a clean separation of utilties over HsSyn, with the generic
  versions in HsUtils, and the specific versions in RdrHsSyn, RnHsSyn
  and TcHsSyn as appropriate.

- Remove the RdrBinding data type, which was really just a nested list
  with O(1) append, and use OrdList instead.  This makes it much clearer
  that there's nothing strange going on.

- Various other minor cleanups.