Automatically run Loop at end of program (autorun)

[clue]

When using the Loop class, it will automatically execute the loop at the end of
the program. This means the following example will schedule a timer and will
automatically execute the program until the timer event fires:

use React\EventLoop\Loop;

Loop::addTimer(1.0, function () {
    echo 'Hello' . PHP_EOL;
});

As of v1.2.0, we highly recommend using the Loop class this way and omitting any
explicit run() calls. For BC reasons, the explicit run()
method is still valid and may still be useful in some applications, especially
for a transition period towards the more concise style.

If you don't want the Loop to run automatically, you can either explicitly
run() or stop() it. This can be useful if you're using
a global exception handler like this:

use React\EventLoop\Loop;

Loop::addTimer(10.0, function () {
    echo 'Never happens';
});

set_exception_handler(function (Throwable $e) {
    echo 'Error: ' . $e->getMessage() . PHP_EOL;
    Loop::stop();
});

throw new RuntimeException('Demo');

This is a pure feature addition that works across all loops and does not affect BC.

Builds on top of #226, #229 and #231

[kelunik]

We've previously briefly discussed this, and I'd recommend against introducing this. Application registered shutdown functions won't be run if they're registered inside event loop watchers and a fatal error or explicit exit; happen, see e.g. https://3v4l.org/duM4S.

[clue]

We've previously briefly discussed this, and I'd recommend against introducing this. Application registered shutdown functions won't be run if they're registered inside event loop watchers and a fatal error or explicit exit; happen, see e.g. https://3v4l.org/duM4S.

The autorun functionality is a new feature that will only be used when switching to the Loop class. Existing code explicitly using the $loop->run() call or new code explicitly switching to Loop::run() can still explicitly run the loop without resorting to the autorun functionality. If this is a concern for your use case, the loop would not be executed from within a shutdown function in this case.

Other than that, I don't see how this would affect real-world use cases in any negative way. I've added a bunch of test cases that verify common application setups work as expected (either using the new style or the existing style). If you feel anything's missing, I'm happy to look into this and provide additional test cases 👍

[clue]

Rebased on master to resolve merge conflict now that #231 is in