Error on negating unsigned types

[Johan Engelen via Digitalmars-d]

On Tuesday, 11 July 2017 at 19:46:00 UTC, Johan Engelen wrote:
> The Weka folks would like to see a compile error on negating 
> unsigned types:

Also this nice hackery would need a workaround:
```
    if ((y&(-y))==y)
```