Utterly minimal changes to collapse datetimetz into datetime, and timetz

Utterly minimal changes to collapse datetimetz into datetime, and timetz
into time.  This is little more than *exporting* the datetimetz object
under the name "datetime", and similarly for timetz.  A good implementation
of this change requires more work, but this is fully functional if you
don't stare too hard at the internals (e.g., right now a type named
"datetime" shows up as a base class of the type named "datetime").  The
docs also need extensive revision, not part of this checkin.