Subtraction is backward on non-Ints

[habnabit]

A test added to int.rs:

    #[test]
    fn int_sub() {
        let l : Int = "100".parse().unwrap();
        let a : Int = "99".parse().unwrap();
        assert_mp_eq!(&l - 1, a);
    }

Fails:

---- int::test::int_sub stdout ----
    assertion failed: &l - 1 == a
thread 'int::test::int_sub' panicked at '101 != 99', src/int.rs:2972

[Aatch]

Well, that's embarassing. I likely made a copy-paste error when implementing the op-assign traits. Fixing it now.