Bug: incorrect JSON serialization on FieldElement

[xJonathanLEI]

So when working on #62 I found out about a critical bug on the FieldElement type that's basically breaking all contract deployments and calls. This code doesn't work:

let num = FieldElement::from_hex_be("0x1").unwrap();
let text = serde_json::to_string(&num).unwrap();
assert_eq!(text, "\"1\"");

Fails with:

thread 'tests::test_json_ser' panicked at 'assertion failed: `(left == right)`
  left: `"\"0\""`,
 right: `"\"1\""`', starknet-ff/src/lib.rs:450:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[xJonathanLEI]

Looks like checked_rem on crypto_bigint::U256 is causing the issue. It's returning 0 for the operation.