Support in-place negation/reciprocal operations

[mozhewen]

Big numbers are often stored in some data structure such as values of a BTreeMap. When I want to do some local change on the data structure, I usually get a &mut of the number. In this case, if I'm taking a neg(), a clone() of the number is needed, which is inefficient since it's essentially only a flip of sign. Similar for recip().

A new function like

fn set_neg(&mut self) {
    self.sign = -self.sign;
}

may be helpful. In fact, I even think it should be a method of the Neg trait.