`BigInt` and `BigUint` should implement `Hash`

[vks]

Something along the lines of

impl<H: std::hash::Writer> std::hash::Hash<H> for BigInt {
    #[inline]
    fn hash(&self, hasher: &mut H) {
        self.data.as_slice().hash(hasher)
    }
}

should work.

[Gankra]

#[deriving(Hash)] should be sufficient in this case, I think.

[Gankra]

Hmm, nope, neither of our solutions are sufficient. Presumably 100 should hash to the same as 000100, but that doesn't hold if you naively hash the whole Vec. Need to truncate blocks that are all 0's at the end of the slice.