Add sort support routine for the UUID data type.

This introduces a simple encoding scheme to produce abbreviated keys:
pack as many bytes of each UUID as will fit into a Datum.  On
little-endian machines, a byteswap is also performed; the abbreviated
comparator can therefore just consist of a simple 3-way unsigned integer
comparison.

The purpose of this change is to speed up sorting data on a column
of type UUID.

Peter Geoghegan