Use register_shutdown_function for panic

[kelunik]

I'm not entirely sure about that one. It prevents us from directly printing to STDERR and instead deferring to PHP to handle that. PHP will automatically choose what to do based on ini settings like display_errors.

One potential downside of this approach is people using exit inside callbacks registered with register_shutdown_function. That would result in hiding the exception.

This might happen if you run the event loop from a register_shutdown_function. RxPHP does this in its examples to reduce the boilerplate for these. In that case our exit(255); here directly exits without executing any further shutdown functions.

Feedback and opinions are very welcome.

[kelunik]

Closing in favor of #45.