Non-negative GCD

[mikem8891]

I think the "non-negative" statement needs to be added to the Integer trait itself. If all implementations do not need to return a non-negative value, then the "non-negative" statement should be removed from num_integer::gcd.

I imagine "non-negative" invariant may be important for num_rational::Ratio<T> to work properly. If we do not add the statement to the Integer trait itself, then a statement may be needed in the num-rational crate.

Tangent:

1. Should lcm include the same statement? As far as I can tell, lcm is implemented to return only non-negative numbers for primitive integers and BigInt.
2. The num-bigint crate has the outdated "positive" wording instead of "non-negative". Should I update that too?