proposal: UTF8 to_ascii function

Hello,

combination functions to_ascii and convert_to is broken now. Problem
is in convert_to function. It doesn't support 8bit output encoding.

Current workaround:

CREATE FUNCTION to_ascii(bytea, name)
RETURNS text AS 'to_ascii_encname' LANGUAGE internal;

SELECT to_ascii(convert_to('Příliš žlutý kůň', 'latin2'),'latin2');

I don't expect column collate for 8.4, so we need to have workable
to_ascii function.

I propose function to_ascii(text, name) that internally convert text
from utf8 encoding when it's necessary.

Regards
Pavel Stehule