Fix byte ordering

[thrsten]

Hi! If I'm not mistaken the Scalar struct seems to represent values internally as little endian (cf. https://docs.rs/curve25519-dalek/3.2.1/curve25519_dalek/scalar/struct.Scalar.html#). After decoding the hex string, I think the bytes need to be reversed before creating the scalar.

For example, in this snippet Scalar would represent 1 as 0x01..00 rather than 0x00..01

let one: u64 = 1;
let s_one = Scalar::from(one);

let bytes_one = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1];
let s_bytes_one = Scalar::from_bytes_mod_order(bytes_one);

let bytes_one_reverse = [1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0];
let s_bytes_one_reverse = Scalar::from_bytes_mod_order(bytes_one_reverse);
    
assert!(s_one != s_bytes_one);
assert!(s_one == s_bytes_one_reverse);

Let me know if you have any questions (or disagree) and thanks for publishing your artifacts!!

Thorsten