Enums patch v2

Hi all

Here is an updated version of the enums patch. It has been brought up to
date and applies against current CVS HEAD. The original email is at [1],
and describes the implementation.

This version contains sgml documentation, and contains the missing
bounds checks and error codes noted in the last email.

As mentioned before, the only part that I'm not super keen on is the
hack required to pass the type oid in to the text-to-enum cast function,
since normally those take type mods but not type oids. I wasn't sure how
else to get a cast working though, short of allowing another type of
cast function which accepts type oids as well. That seemed overkill for
just one case, though, and was getting a bit beyond the realms of what I
wanted to get done with this patch.

Anyway, it's reasonably feature complete and should be appropriately
documented now, so feedback gratefully accepted.

Cheers

Tom

[1] http://archives.postgresql.org/pgsql-patches/2006-09/msg00000.php