Change to use functional API instead of static Util class

[clue]

The raising of the dead part 5.

This changeset updates the code base to use a functional API instead of a static Util class.

// oldReact\Async\Util::parallel(…); React\Async\Util::series(…); React\Async\Util::waterfall(…); // newReact\Async\parallel(…); React\Async\series(…); React\Async\waterfall(…);

Refs clue/reactphp-block#13, reactphp/promise#12 and others
Builds on top of #2, #3, #4 and #5