Ordering for Dec type is buggy

[brentstone]

The ordering for Dec types does not work properly for large numbers it seems. An example encountered in the ledger can be summarized with the following lines:

let smaller = Dec::from_str("6483947304.195066085701").unwrap();
let larger = Dec::from_str("32418116583.390243854642").unwrap();
assert!(smaller < larger);

The above assertion fails.

[brentstone]

@batconjurer

[batconjurer]

So the Ord implementation for I256 accidentally inherited from the Ord implementation on [u64; 4], not Uint([u64; 4]) which is an issue since the Uint type is little endian.

[batconjurer]

But the ordering would be correct if only the lowest u64 word was used.