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.

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

// new
React\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