Solve Pell's equation
```
use rug::Integer;
let v = pell_equation::solve_pell(Integer::from(2));
assert_eq!(v, pell_equation::Solution::Negative(Integer::from(1), Integer::from(1)));
let w = pell_equation::solve_pell(Integer::from(3));
assert_eq!(w, pell_equation::Solution::Positive(Integer::from(2), Integer::from(1)));
```