Encoding of ark_secp256r1 Scalars must be big-endian

[armfazh]

The NargSerialize trait stipulates that the encoding of field elements must follow I2OSP (big-endian).

However, the implementation for ark_ff objects was done using little-endian.

This script shows the mismatch.

use spongefish::Encoding;
type ArkP256Scalar = <ark_secp256r1::Projective as ark_ec::PrimeGroup>::ScalarField;
let ark_scalar = ArkP256Scalar::from(18u64);
let encoded = ark_scalar.encode();
let big_endian = [vec![0u8; 31], vec![18]].concat();
assert_eq!(encoded.as_ref(), big_endian, "mismatch endianess")