BUG: Integer overflow warning applies to scalars but not arrays

Simple demo:

```python
In [1]: np.uint8(0) - np.uint8(1)
RuntimeWarning: overflow encountered in ubyte_scalars
Out[1]: 255

In [2]: np.uint8(0)[...] - np.uint8(1)[...]
Out[2]: 255
```

Should we even warn at all for unsigned integers?