Use interleaving to improve Fp multiplication

This only changes the `Mul` impl, which is non-const.

Author: str4d

src/fp.rs

@@ -151,7 +151,7 @@ impl<'a, 'b> Mul<&'b Fp> for &'a Fp {
 
     #[inline]
     fn mul(self, rhs: &'b Fp) -> Fp {
-        self.mul(rhs)
+        Fp::sum_of_products([*self], [*rhs])
     }
 }