v4.1.0

• 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)