User defined I/O conversion casts

Since 8.3, the system provides automatic I/O conversion casts between
the built-in string types and other types, but there's currently no way
for a user to declare additional casts using the I/O functions. You can
always create a simple SQL function to do that, but it seems like we
should provide a direct interface for that. I propose the syntax:

CREATE CAST (<source> AS <target>) WITH INOUT [AS IMPLICIT | AS ASSIGNMENT]

Conveniently, INOUT is already a col_name_keyword, so this works without
any conflicts or new keywords.

This would be very useful for those who want relax some built-in
automatic assignment casts to implicit casts, 8.2 style, as well as
anyone creating a user-defined data type with casts that can be
implemented with the I/O functions.

Patch attached. I'm using a magic OID "1" in pg_cast.castfunc field to
mark these extra I/O conversion casts. Perhaps we want to have a
separate field for that or something, but that was a quick way to get
started.

Anyone else think this is a good idea? Thoughts on the implementation?

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com