Unpacking syntax

[Timon Gehr]

On 9/6/24 00:08, Timon Gehr wrote:
> These are my ideas for how to support unpacking any tuple-like type into 
> multiple components.
> 
> ...

Jacob asked on Discord whether in a `foreach` statement, key and value 
can both be unpacked, and whether in a foreach statement, `(const x, y)` 
is also allowed. Both of those work. In particular, this compiles and 
runs with my `unpacking` branch of DMD:

```d
import std.typecons: t=tuple, T=Tuple;

void main(){
     auto aa=[t(1, 2): t(3, 4), t(5, 6): t(7, 8)];
     foreach((const k0, k1), (v0, ref v1); aa){
         k1 = k0;
         v1 = v0;
         v0 = 22;
     }
     assert(aa == [t(1, 2): t(3, 3), t(5, 6): t(7, 7)]);
     foreach((const x, y); [t(1, 2), t(2, 3)]){}
}
```