Returntypehinting RFC

RFC'd by @morrisonlevi

PR'd for review + CI

https://wiki.php.net/rfc/returntypehinting
```
it doesn't actually mention replacing the function keyword with the type in my RFC
```
Isn't ```return_type function_name()``` actually that mention? If you meant ```return_type function function_name()``` then please edit the RFC to say that, because otherwise it is confusing as to what you mean.

Also,  of course, the fact that old RFC was rejected doesn't mean it was rejected because of the syntax. We had syntax RFCs (like ```[]```) rejected a number of times, but then accepted.