Returntypehinting RFC

RFC'd by @morrisonlevi

PR'd for review + CI

https://wiki.php.net/rfc/returntypehinting
This is how HHVM has it implemented. We have discussed this issue
previously on the list, and people seemed slightly in favor of putting
the return type after the use statement, because this:

    function ($value) : Bar use ($Baz) {

looks like Bar uses Baz somehow. I am not fond of either syntax,
honestly; aside the reasons you just presented, can you think of other
reasons for doing it this way?