Trait that adds a bound function.
Automatically implemented for anything that is `Ord`
`bound(&self, lower: &Self, upper: &Self) -> &Self`
```rust
use num_bound::Bound;
let upper = 500;
let lower = 200;
assert_eq!(300.bound(&lower, &upper), &300);
assert_eq!(550.bound(&lower, &upper), &500);
assert_eq!(100.bound(&lower, &upper), &200);
```