extra::bigint does not handle overflow correctly

[emberian]

As an example,

extern mod extra;

use extra::bigint;
use extra::bigint::BigInt;
use extra::bigint::FromStr;
fn main() {
    let a:bigint::BigInt = FromStr::from_str("31").unwrap();
    let b:bigint::BigInt = FromStr::from_str("999999999999999999999999999999999999").unwrap();
    let c = (a+b).to_str();
    println(format!("{:?}",c));
}

will fail with

task '<main>' failed at 'vector size is too large: 18446744073709551615', /build/rust-git/src/rust/src/libstd/vec.rs:191

bigint needs to be carefully combed for overflow-correctness.

There is a test that makes sure this fails rather than segfaults in src/test/run-pass/bigint-overflow-segfault.rs that will need to be updated.