This can be used as a checksum for unordered sets.  bit_and and bit_or
already exist.

Author: Alexey Bashtanov <bashtanov@imap.cc>
Reviewed-by: Ibrar Ahmed <ibrar.ahmad@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/9d4582ae-ecfc-3a13-2238-6ab5a37c1f41@imap.cc