Missing BigInt::pow() + BigUInt::pow()

[JamesMcGuigan]

There are a number of missing functions when comparing num-bigint-dig with num-bigint.

The most noticeable is BigInt::pow(), which is strange because BigInt::modpow() is implemented.

use num_traits::{Num, One, Pow, Signed, Zero};
impl BigInt {
    /// Returns `self ^ exponent`.
    pub fn pow(&self, exponent: u32) -> Self {
        Pow::pow(self, exponent)
    }
}

• https://docs.rs/num-bigint/0.4.3/num_bigint/struct.BigInt.html#method.pow
• https://docs.rs/num-bigint/0.4.3/src/num_bigint/bigint.rs.html#1012-1014

Unsure how far the codebases have diverged, but it might be worth doing an API review between the two projects (ideally num-bigint-dig should be a drop-in replacement for num-bigint) and potentially performing an unstream merge for any safe new features since the original fork.