Add powi method for `Sign` enum

[feefladder]

The multiplication method is provided, but raising a sign to an integer power is also a common use-case I think?

#[inline]
fn sign_powi<T: Integer>(sign: Sign, pow: T) -> Sign {
  if pow.is_odd() {sign} else {Sign::Plus}
}

or in an impl:

#[inline]
impl Sign {
  fn powi<T: Integer>(&self, pow: T) -> Sign {
    if pow.is_odd() {*self} else {Sign::Plus}
  }
}

I came here, because fraction crate uses Sign, which I use.

[cuviper]

It's not quite that simple since there's also NoSign, but we do have an internal function for this already:

num-bigint/src/bigint/power.rs

Lines 9 to 21 in 3674819

	/// Help function for pow
	///
	/// Computes the effect of the exponent on the sign.
	#[inline]
	fn powsign<T: Integer>(sign: Sign, other: &T) -> Sign {
	if other.is_zero() {
	Plus
	} else if sign != Minus || other.is_odd() {
	sign
	} else {
	-sign
	}
	}

I was hesitant to add public Sign powers during the initial Pow review, #54 (comment), and I'm still not sure about it. It's not really the intention for Sign to act like a complete mathematical entity on its own. Can you explain more about why fraction would want this? It looks like they have their own Sign anyway.