Implement `power` with float exponents

[PgBiel]

Currently, int.power(a, b) and float.power(a, b) use recursive implementations, and thus only support integers for b. As an exception, I've explicitly added an algorithm for square root taken from nix-math, which applies when b == 0.5, so that the square_root function still works. Still, would be nice to have a more general algorithm, which can be hard as Nix doesn't have built-in power.