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;
asserteq!(300.bound(&lower, &upper), &300); asserteq!(550.bound(&lower, &upper), &500); assert_eq!(100.bound(&lower, &upper), &200);
```