flagging unsigned subtraction assigned to bigger signed number?

[Richard (Rikki) Andrew Cattermole]

Counter proposal: throw data flow analysis at it, rather than change the 
language.

It would be able to catch things like:

```d
int[] array = [1, 2];
size_t offset;

foreach(i; 0 .. array.length) {
	offset = i + 1;
}

array[offset]; // Error out of bounds!
```

But not:

```d
void func(int[] array, size_t offset) {
	array[offset];
}
```

To catch the second would require false positives to be acceptable, and 
the default pain tolerance for the D community is lower.

Yes this is more complex to implement than a language change, but the 
support can be improved over time, it isn't fixed, and we can have the 
slower DFA that will error out in the latter example.