Function decrypt 

pub fn decrypt<W: Word>(ct: [W; 2], key: &Vec<u8>, rounds: usize) -> [W; 2]

Decrypts a ciphertext ct and returns a plaintext pt. The ct is an array of two words W (u8, u16, u32, u64, u128)

Example:

use rc5_cipher::decrypt;

let rounds = 12;
let key = vec![0x00, 0x01, 0x02, 0x03];
let ct  = [0x21u8, 0x2A];

let pt = decrypt(ct, &key, rounds);

assert_eq!(pt, [0x00u8, 0x01]);