Assertions in PL/PgSQL

Hi,

Attached is a patch for supporting assertions in PL/PgSQL. These are
similar to the Assert() backend macro: they can be disabled during
compile time, but when enabled, abort execution if the passed expression
is not true.

A simple example:

CREATE FUNCTION delete_user(username text) RETURNS VOID AS $$
BEGIN
DELETE FROM users WHERE users.username = delete_user.username;
ASSERT FOUND;
END
$$ LANGUAGE plpgsql;

SELECT delete_user('mia');
ERROR: Assertion on line 4 failed
CONTEXT: PL/pgSQL function delete_user(text) line 4 at ASSERT

Again, I'll add this to the open commitfest, but feedback is greatly
appreciated.

Regards,
Marko Tiikkaja