Implement the 'checked_pow' function for num::rational::Ratio as is done for i32, i64 etc..

Currenly if a [num::rational::Ratio](https://docs.rs/num/latest/num/rational/struct.Ratio.html) encouters a numerical error in a call to the 'pow' method it raises a panic. The fundamental Rust numerical data types implement [checked_pow](https://doc.rust-lang.org/std/primitive.i32.html#method.checked_pow). It would be useful it `num::rational::Ratio` implemented an equivalent method.