pgsql: Add bit_xor aggregate function

Add bit_xor aggregate function

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: /messages/by-id/9d4582ae-ecfc-3a13-2238-6ab5a37c1f41@imap.cc

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f9a0392e1cf36179c984f655a6589734b8dd7468

Modified Files
--------------
doc/src/sgml/func.sgml | 27 +++++++++++++++++++++++++++
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_aggregate.dat | 8 ++++++++
src/include/catalog/pg_proc.dat | 12 ++++++++++++
src/test/regress/expected/aggregates.out | 23 +++++++++++++++--------
src/test/regress/sql/aggregates.sql | 12 ++++++++++--
6 files changed, 73 insertions(+), 11 deletions(-)