Add CI to pg_hexedit using cirrus-ci.org

[petergeoghegan]

When Travis CI support was dropped, we never replaced it with another free CI provider. That should happen at some point, and should test all Postgres branches for major versions that are under support by the Postgres community (just as it was back when Travis CI could still be used).

Presumably we'd want to use cirrus-ci.org for this, since that's what Postgres itself uses.