Returntypehinting RFC

RFC'd by @morrisonlevi

PR'd for review + CI

https://wiki.php.net/rfc/returntypehinting
@smalyshev's comment questions the fundamental position of the return type hint (to put it before the function declaration instead of after, as in e.g. Java); i.e.:

```diff
-function ($value) use ($multiplier) : array {
-function ($value) : array use ($multiplier) {
+array function ($value) use ($multiplier) {
```

To be honest, that was my very first reaction to the overall change as well.  However, the RFC seemingly states that this ship has sailed already (so I didn't feel bold enough to remotely mention that fundamentally different possibility).