Returntypehinting RFC

RFC'd by @morrisonlevi

PR'd for review + CI

https://wiki.php.net/rfc/returntypehinting
> I don't see how it is "sailed already"

I hope you _did_ read the [RFC](https://wiki.php.net/rfc/returntypehinting#position_of_type_declaration), which prominently states the following:

>  The former position has been proposed in the past and the RFCs were either declined or withdrawn. One cited issue is that many developers wanted to preserve the ability to search for `function foo` to be able to find the definition for foo.
> 
> The latter position is used in several languages; it is also used in C++11's lambda declarations when the type cannot be inferred by the compiler.
> 
> Declaring the return type after the parameter list has no conflicts in the parser. 

In short, the fundamental before/after position was seemingly discussed to great lengths already and the authors/editors of the RFC seemingly had to pull a hammer to stop a bikeshed.

If you want to re-open that discussion, please do so on the php-internals list, but ensure to provide substantial arguments, _and_ despite doing so, be prepared for some good level of slashback. — It might be worth to do so (especially in light of the original Closure import topic here).  I'm new to php-internals, so I certainly don't have any karma to burn and won't do so. ;)

All I'm questioning here is the consistency of function signatures with regard to Closures that are importing variables, based on status quo, and actively taking above mentioned hammer into account. :)