Returntypehinting RFC

RFC'd by @morrisonlevi

PR'd for review + CI

https://wiki.php.net/rfc/returntypehinting
> One language of interest is Hack which also uses a colon here.

That argument is most likely the most sensible and most compelling one, thanks.

I was mainly concerned about (1) needless visual clutter, and also (2) requiring user-space Tokenizer-based code parsers to additionally account for the (optional) `T_WHITESPACE`, `":"`, (optional) `T_WHITESPACE` sequence.

It's a minor issue.  However, since it is reasonable to expect that we'll see an adoption of this grammar in >80% of all PHP code in the future (probably 100% after scalar type hints are in), I thought it would be worth to raise the question.

Given that this can be considered a deliberate choice, clarifying it in the RFC would be welcome.

Thanks! - Really looking forward to use this! :-)