Bug: Modpow -1^2 == -1

[jounathaen]

The following code calculates a^i for i = 1,2,3,4:

extern crate num_bigint;
extern crate num_traits;
use num_bigint::BigInt;

fn main()
{
    let a = BigInt::from(-1);
    let m = BigInt::from(10);
    
    println!("a = {}", a);
    let a_pow_1 = a.modpow(&BigInt::from(1), &m);
    println!("a^1 mod 10 = {}", a_pow_1);
    let a_pow_2 = a.modpow(&BigInt::from(2), &m);
    println!("a^2 mod 10 = {}", a_pow_2);
    let a_pow_3 = a.modpow(&BigInt::from(3), &m);
    println!("a^3 mod 10 = {}", a_pow_3);
    let a_pow_4 = a.modpow(&BigInt::from(4), &m);
    println!("a^4 mod 10 = {}", a_pow_4);
    
    let a_times_a = (&a * &a) % &m;
    println!("a*a mod 10 = {}", a_times_a);
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=7de58584f7dff3342bf0c8cce191bfb7

Expected Output:

a = -1
a^1 mod 10 = 9
a^2 mod 10 = 1
a^3 mod 10 = 9
a^4 mod 10 = 1
a*a mod 10 = 1

Real Output:

a = -1
a^1 mod 10 = 9
a^2 mod 10 = 9
a^3 mod 10 = 9
a^4 mod 10 = 9
a*a mod 10 = 1

I don't know, if my understanding of modulo calculus is correct, but from all I know, I'd assume, that -1^2 == 1 and that a * a == a^2 regardless of the modulus.

[cuviper]

Sorry, this was fixed by #114, but I had neglected to publish a new release. Please try 0.2.4.