Returntypehinting RFC

RFC'd by @morrisonlevi

PR'd for review + CI

https://wiki.php.net/rfc/returntypehinting
Just as a clarification, the RFC says that previous RFCs put the return type before the function name and they've all been withdrawn or declined; it doesn't actually mention replacing the function keyword with the type in my RFC. However, it has been talked about as part of the official discussion on the mailing list, as well as many times off of it.

This week I have been redoing the implementation and basing it off of master. I also intend to edit parts of the RFC to explain things in a more precise way, and will definitely talk more about the position of the return type.