PostgreSQL 13 support

[dmitryuk]

Hey guys! Do you plan to support the project? For example, the extension works with PostgreSQL up to 11 version, but current is already 13.

[afiskon]

@dmitryuk I've just tested the extension on the upstream version of PostgreSQL. It seems to work just fine:

Aleksanders-MacBook-Pro:zson eax$ make install
/bin/sh /Users/eax/pginstall/lib/postgresql/pgxs/src/makefiles/../../config/install-sh -c -d '/Users/eax/pginstall/share/postgresql/extension'
/bin/sh /Users/eax/pginstall/lib/postgresql/pgxs/src/makefiles/../../config/install-sh -c -d '/Users/eax/pginstall/share/postgresql/extension'
/bin/sh /Users/eax/pginstall/lib/postgresql/pgxs/src/makefiles/../../config/install-sh -c -d '/Users/eax/pginstall/lib/postgresql'
/usr/bin/install -c -m 644 .//zson.control '/Users/eax/pginstall/share/postgresql/extension/'
/usr/bin/install -c -m 644 .//zson--1.1.sql .//zson--1.0--1.1.sql  '/Users/eax/pginstall/share/postgresql/extension/'
/usr/bin/install -c -m 755  zson.so '/Users/eax/pginstall/lib/postgresql/'
Aleksanders-MacBook-Pro:zson eax$ make installcheck
/Users/eax/pginstall/lib/postgresql/pgxs/src/makefiles/../../src/test/regress/pg_regress --inputdir=./ --bindir='/Users/eax/pginstall/bin'    --dbname=contrib_regression zson
(using postmaster on Unix socket, default port)
============== dropping database "contrib_regression" ==============
DROP DATABASE
============== creating database "contrib_regression" ==============
CREATE DATABASE
ALTER DATABASE
============== running regression test queries        ==============
test zson                         ... ok          231 ms

=====================
 All 1 tests passed.
=====================

Please let us know if the problem is still actual. If it is please provide a little more details on the problem you have.