Feature: Support PHP 8.1, avoid deprecation warning concerning \Countable::count(...) return type. (#32 by @bartvanhoutte)

Improve documentation and simplify examples by updating to new default loop. (#27 and #29 by @PaulRotmann and #30 by @SimonFrings)

Improve test suite to use GitHub actions for continuous integration (CI). (#28 by @SimonFrings)