new DIP39: Safe rvalue references: backwards compatible, safe against ref/nonref code evolution, compatible with UFCS and DIP38

[timotheecour]

> OK, First remark on the way the DIP is presented. You start 
> right away with example which make it hard to understand o 
> general big picture.

I've clarified differences with the rejected DIP36 in the 
abstract and added more context.

> Second, ^ is not unused in D and the DIP introduce an ambiguity 
> here.

Indeed, it's used in XOR; I've clarified this point. It should 
still be unambiguous as XOR is binary.

> Making the temporary creation explicit is an idea that hasn't 
> been explored that much. I have to think more about it.

Could you point out to any potential pitfalls, when used as 
described in section 'Implementation details' ?


I've also incorporated a useful suggestion from Dmitry S, which 
is to allow LV^ to create a temporary from an lvalue reference, 
as oppose to result in compile error in that case, see the 
updated DIP.