Rewrite uuid input and output routines to avoid dependency on the

nonportable "hh" sprintf(3) length modifier. Instead, do the parsing
and output by hand. The code to do this isn't ideal, but this is
an interim measure anyway: the uuid type should probably use the
in-memory struct layout specified by RFC 4122. For now, this patch
should hopefully rectify the buildfarm failures for the uuid test.

Along the way, re-add pg_cast entries for uuid <-> varchar, which
I mistakenly removed earlier, and bump the catversion.