BoxedUint serialization portability issue across architectures

[elichai]

When using BoxedUint with a precision that isn't a multiple of 64 (or closer to that than a multiple of 32) then serializing on a 64 bit machines will fail deserializing on 32 bit machine, and if used in a hashing scenario they'll have incompatible hashing,

Issue 1, using to_*bytes()
On 64 bit machines:

let one = BoxedUint::one_with_precision(32);
assert_eq!(&*one.to_le_bytes(), &[1, 0, 0, 0, 0, 0, 0, 0]);

on 32 bit machines:

let buf = [1, 0, 0, 0, 0, 0, 0, 0];
let back = BoxedUint::from_le_slice(&buf, 32);
assert_eq!(back, Err(DecodeError::InputSize));

When using serde, the problem is increasing the bit precision.

On 64 bit machines:

let one: BoxedUint = BoxedUint::one_with_precision(32);
let bincoded = bincode::serde::encode_to_vec(&one, bincode::config::standard()).unwrap();
assert_eq!(bincoded, vec![8, 1, 0, 0, 0, 0, 0, 0, 0]);

On 32 bit machines:

let one = BoxedUint::one_with_precision(32);
let from_network = [8, 1, 0, 0, 0, 0, 0, 0, 0]; // bincode of `1` in 64 bit machines.
let (one_network, _) = bincode::serde::decode_from_slice::<BoxedUint, _>(&from_network,bincode::config::standard()).unwrap();
assert_eq!(one.bits_precision(), 32);
assert_eq!(one_network.bits_precision(), 64);

You can see that now they have incompatible bits precision even though they were both initialized with BoxedUint::one_with_precision(32) and things like BoxedUint::add_mod will fail