Returntypehinting RFC

RFC'd by @morrisonlevi

PR'd for review + CI

https://wiki.php.net/rfc/returntypehinting
@smalyshev I tend to agree for unnamed functions (Closures). (But still, array() followed by brackets (for function body) is not clashing with existing syntax).

Not sure if we really should add return type hinting to Closures. (I was told it'd be needed for eventual typing of callables later; but not sure if I'd want that at all)