The new `f4jumble_mut` and `f4jumble_inv_mut` functions take a `&mut [u8]`, and return a new `Error` type if the slice has an invalid length. The original `f4jumble` and `f4jumble_inv` functions are now more efficient as they only allocate once.
Co-authored-by: Daira Hopwood <daira@jacaranda.org>