Returntypehinting RFC

RFC'd by @morrisonlevi

PR'd for review + CI

https://wiki.php.net/rfc/returntypehinting
I don't see how it is "sailed already", any syntax is up to discussion unless it can not be implemented or collides with existing one. 

@bwoebi replacing function keyword won't work since array($value) already has a meaning in PHP. Besides that, it won't be really clear what's going on.