Add filter function

I don't see a way to do the following (easily) without resorting to phobos
```
import mir.ndslice.slice: sliced;
import mir.math.stat: mean;
import std.algorithm: filter;

void main() {
    auto x = [0.0, 1, 2, 3, 4, 5, 6].sliced;
    auto y = x.filter!(a => a < 3).mean;
    assert(y == 1);
}
```