Fuel update for UTCDateAndTime

The Fuel serializer is loadable in Squeak 5.2 from the SqueakMap loader,
and is supported for Pharo, Squeak, and Cuis (great kudos to the
maintainers for doing this).

In order to change DateAndTime to a simpler UTC-based implementation in
Squeak, with two instance variables rather than four, an update will be
needed for Fuel to understand the new format.

The attached change set provides the necessary support, while maintaining
compatibility with the traditional DateAndTime and interoperability
between images that support either format. For example, an image can
serialize a traditional DateAndTime with Fuel, and another image with
UTC DateAndTime can materialize it in the new UTC format.

I am not yet sure how to submit this to the Fuel maintainers, so I'm
posting it here as a change set for now.

Dave