Function decode 

pub fn decode(data: &str) -> Result<Vec<u8>, DecodeBase32Error>

Example:

use otp::encoding::base32;

let hello_world = "JBSWY3DPFQQHO33SNRSCC";
let decoded = base32::decode(hello_world).expect("Decoding failed");
assert_eq!(b"Hello, world!", decoded.as_slice());
assert_eq!(hello_world, base32::encode(b"Hello, world!"));

let hello_world_with_pad = "JBSWY3DPFQQHO33SNRSCC===";
let result_invalid_err = base32::decode(hello_world_with_pad);
assert!(matches!(result_invalid_err, Err(base32::DecodeBase32Error::InvalidChar(_))));