Add signum

Would be nice to add this:

```javascript
// returns +/-1 giving the number's sign, except when value is zero, in which +/-0 is returned
P.signum = function () {
    return !!this.c && this.c[0] == 0 ? this.s*0 : this.s;
};
```

(May be a better way to just return the signed zero value)