Returntypehinting RFC

RFC'd by @morrisonlevi

PR'd for review + CI

https://wiki.php.net/rfc/returntypehinting
@sun I definitely prefer that syntax. To me, and probably any C developer, it is obvious to have the return type before the function. It doesn't break searching for "function whatever" and it doesn't get in the way of "use" nor require some weird colon syntax which looks completely foreign to me.