Tick callback receives no arguments, fix cyclic dependency

[clue]

The tick API has been documented as part of #100, however it makes no mention of how the tick function should look like. It actually currently receives the loop instance, which is entirely undocumented. The loop parameter also causes an unneeded cyclic dependency which hinders garbage collection.

This simple PR changes it so that the tick callback no longer receives a loop argument. This is obviously a small BC break, so this PR ensures to provide some upgrade guides to those affected by this. This is now also in line with the timer API, see #102.

Builds on top of #100 and refs #102

Remove loop argument and suggest using closure binding instead

// old (loop passed as parameter)
$loop->futureTick(function ($loop) {
    $loop->stop();
});

// already supported before: use closure binding as usual
$loop->futureTick(function () use ($loop) {
    $loop->stop();
});

[clue]

@WyriHaximus Thanks for spotting, updated and squashed!

[WyriHaximus]

@clue LGTM