Converting to usize/isize?

I'm not a Rust veteran, but is there a (fixable) reason why the `ux` types can't be converted directly to `usize`/`isize`?

```rust
let x = usize::from(u4::new(0)); // the trait bound `usize: std::convert::From<ux::u4>` is not satisfied
let x = usize::from(u8::from(u4::new(0))); // works, but is obviously not ergonomical
```