DIP19: Remove comma operator from D and provision better syntactic support for tuples

[Adam D. Ruppe]

I'm not for removing the comma operator, but it occurs to me we 
could do it in the library:

auto commaOperatorReplacement(T...)(T t) {
    return t[$-1];
}

There might be some edge case where that wouldn't work, but I 
think it works in most cases.