Drop deprecated Factory, use Loop instead

[clue]

This simple changeset drops the deprecated Factory class from this package. As of v1.2.0, we recommend using the Loop class instead, so upgrading should be straightforward for most use cases:

// old (no longer supported)
$loop = React\EventLoop\Factory::create();

// new (already supported since v1.2.0)
$loop = React\EventLoop\Loop::get();

The existing factory logic has been refactored slightly and has been moved into the existing Loop. I've tried to keep changes to a minimum, so this does not otherwise affect the public Loop API. This PR also builds on top of recent PR #273 that dropped some of the legacy loop implementations.

The original deprecation message has been in place for a couple of years already, so this PR builds on top of #226, #229 and #231.

[WyriHaximus]

🎉