Change ??A .sort() methods to return self

[Austin_Hastings]

Presently, the RPA .sort method is inherited from FPA (this is okay). But the method when called does not return anything.

I would like to be able to write, in NQP, something like:

for %hash.keys.sort { 

    say($_); 

}

Changing the return value from nothing (currently) to self seems like a low-impact change that offers decent value.

[Austin_Hastings]

Note: RPA = ResizablePmcArray, and FPA = FixedPmcArray

[NotFound]

Done in r43588