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

[David Piepgrass]

> The built-in tuple is also quite useful when defining templates.
>
> In essence, we have two kinds of tuples: the built-in language 
> tuple is the "unpacked" tuple while Phobos hosts the "packed" 
> one. They each have their own use case and they can coexist 
> peacefully. But the language itself needs to standardize on one 
> or the other.

+1, and it should standardize on "packed" (non-expanded) tuples 
because "unpacked" ones have very unusual behavior, and because 
it's impractical to eliminate "packed" tuples but practical to 
eliminate "unpacked" ones. "unpacked" tuples should only exist as 
an intermediate result (the result of .expand).

> If the language made T… a packed tuple instead, then we could 
> use the packed tuple everywhere and unpack it where necessary, 
> and something like this could be used to make a packed tuple:
>
> 	T getThings(T...)(T.expand t)
> 	{
> 		return T(t);
> 	}
>
> 	T t1;
> 	T t2 = getThings!(T)(t1.expand);

"T.expand" naturally has the connotation "unpacked" to me, 
whereas what you really want to do is indicate that "t" is 
packed, right? Clearly, the syntax for a varargs template like 
this would have to change to indicate that T is non-expanded; 
unfortunately, I don't have a really compelling syntax to suggest.

P.S. If non-expanded tuples were the default, they should 
probably have a quicker syntax than "t.expand" to expand them. I 
suggest overloading unary * as in "*t"; this is known as the 
"explode" operator in boo.