Returntypehinting RFC

RFC'd by @morrisonlevi

PR'd for review + CI

https://wiki.php.net/rfc/returntypehinting
@sun I *did* read the RFC. Unlike you, I read it with attention. It says that replacing "function" with type, namely ```return_type function_name()``` is not working, since people like "function" keyword. I agree with that. It says nothing about prepending the return type to the function keyword. Which is what I am proposing. I think it would be much more productive to discuss the actual proposals.

As for "pulling a hammer", I don't think a single person proposing an RFC has any right to "pull a hammer" on a syntax of a language used by millions.