Feature: Add new delay() function to delay program execution. (#69 by @clue)

echo 'a';
Loop::addTimer(1.0, function () {
    echo 'b';
});
React\Async\delay(3.0);
echo 'c';

// prints "a" at t=0.0s
// prints "b" at t=1.0s
// prints "c" at t=3.0s

Update test suite, add PHPStan with max level and report failed assertions. (#66 and #76 by @clue and #61 and #73 by @WyriHaximus)