Deprecation: foreach: loop index implicitly converted from size_t to int

[BoQsc]

A horrible alternative would be to use `alias` on `size_t` to 
make up a new pseudo-type that is more aligned with the code 
logic.

```
alias integer = size_t;
import std.stdio : writefln;

void main() {
     auto arr = [
         [5, 15],      // 20
         [2, 3, 2, 3], // 10
         [3, 6, 2, 9], // 20
     ];

     foreach (integer i, row; arr)
     {
         double total = 0.0;
         foreach (e; row)
             total += e;

         auto avg = total / row.length;
         writefln("AVG [row=%d]: %.2f", i, avg);
     }
}
```