provide fast NOT cryptographycally-secure checksum functions
for TEXT, results being of INT2, INT4 and INT8 types.

load with:

	sh> psql < <path-to-postgresql>/share/contrib/pgc_checksum.sql

use as:

	psql> SELECT cksum2('some text');
	psql> SELECT cksum4('some text');
	psql> SELECT cksum8('some text');

An empty text results in hash value 0.
A NULL value results in some predefined value.
