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

[bearophile]

Caligo:

> If tuples are ever introduced, I hope parentheses will not be 
> used.
>
> I would prefer something like this:
>
> tuple<2,1,8>

That both breaks code, doesn't improve the syntax, but makes it 
worse.

Bye,
bearophile