Implement "date/time grand unification".

 Transform datetime and timespan into timestamp and interval.
 Deprecate datetime and timespan, though translate to new types in gram.y.
 Transform all datetime and timespan catalog entries into new types.
 Make "INTERVAL" reserved word allowed as a column identifier in gram.y.
 Remove dt.h, dt.c files, and retarget datetime.h, datetime.c as utility
  routines for all date/time types.
 date.{h,c} now deals with date, time types.
 timestamp.{h,c} now deals with timestamp, interval types.
 nabstime.{h,c} now deals with abstime, reltime, tinterval types.
Make NUMERIC a known native type for purposes of type coersion. Not tested.