Support alternate storage scheme of 64-bit integer for date/time types.

 Use "--enable-integer-datetimes" in configuration to use this rather
 than the original float8 storage. I would recommend the integer-based
 storage for any platform on which it is available. We perhaps should
 make this the default for the production release.
Change timezone(timestamptz) results to return timestamp rather than
 a character string. Formerly, we didn't have a way to represent
 timestamps with an explicit time zone other than freezing the info into
 a string. Now, we can reasonably omit the explicit time zone from the
 result and return a timestamp with values appropriate for the specified
 time zone. Much cleaner, and if you need the time zone in the result
 you can put it into a character string pretty easily anyway.
Allow fractional seconds in date/time types even for dates prior to 1BC.
Limit timestamp data types to 6 decimal places of precision. Just right
 for a micro-second storage of int8 date/time types, and reduces the
 number of places ad-hoc rounding was occuring for the float8-based types.
Use lookup tables for precision/rounding calculations for timestamp and
 interval types.  Formerly used pow() to calculate the desired value but
 with a more limited range there is no reason to not type in a lookup
 table. Should be *much* better performance, though formerly there were
 some optimizations to help minimize the number of times pow() was called.
Define a HAVE_INT64_TIMESTAMP variable. Based on the configure option
 "--enable-integer-datetimes" and the existing internal INT64_IS_BUSTED.
Add explicit date/interval operators and functions for addition and
 subtraction. Formerly relied on implicit type promotion from date to
 timestamp with time zone.
Change timezone conversion functions for the timetz type from "timetz()"
 to "timezone()". This is consistant with other time zone coersion
 functions for other types.
Bump the catalog version to 200204201.
Fix up regression tests to reflect changes in fractional seconds
 representation for date/times in BC eras.
All regression tests pass on my Linux box.