Deprecate Readline and move all methods to Stdio

[clue]

Move all public methods to Stdio as is, so the recommend way to call these methods now looks like this:

// old
$stdio->getReadline()->setPrompt('> ');

// new
$stdio->setPrompt('> ');

The original methods continue to exist and work unchanged until the next major release, at which point the Readline class will potentially be removed.