rvalue references

[Diggory]

>
> scope ref T func(scope ref T t) { return t; }
>
> I think this solves the problem.

It doesn't for several reasons:
- You're completely changing the meaning of "scope" for one 
specific case
- This will break DIP25A which only works if scope values cannot 
be returned
- "scope" binds to the function rather than the return type, so 
in this case is equivalent to making the hidden "this" pointer a 
"scope" parameter.