cleaner: reuse .pow() code

src/field/prime/mod.rs

@@ -114,19 +114,12 @@ impl<const P: usize> const FiniteField for PrimeField<P> {
  if self.value == 0 {
  return None;
  }
- let exponent = Self::ORDER - 2;
- let mut result = Self::ONE;
- let mut base = *self;
- let mut power = exponent;
-
- while power > 0 {
- if power & 1 == 1 {
- result *= base;
- }
- base = base * base;
- power >>= 1;
- }
- Some(result)
+
+ // By fermat's little theorem, in any prime field P, for any elem:
+ // e^(P-1) = 1 mod P
+ // So,
+ // e^(P-2) = e^-1 mod P
+ Some(self.pow(Self::ORDER - 2))
  }
 
  fn pow(self, power: usize) -> Self {