[BUG FIX] constraint: serializeCommitment

[SherLzp]

See the issue here: #645

Change the original code:

func (i *Commitment) SerializeCommitment(privateCommitment []byte, publicCommitted []*big.Int, fieldByteLen int) []byte {

	res := make([]byte, len(privateCommitment)+len(publicCommitted)*fieldByteLen)
	copy(res, privateCommitment)

	offset := len(privateCommitment)
	for j, inJ := range publicCommitted {
		offset += j * fieldByteLen
		inJ.FillBytes(res[offset : offset+fieldByteLen])
	}

	return res
}

to

func (i *Commitment) SerializeCommitment(privateCommitment []byte, publicCommitted []*big.Int, fieldByteLen int) []byte {

	res := make([]byte, len(privateCommitment)+len(publicCommitted)*fieldByteLen)
	copy(res, privateCommitment)

	offset := len(privateCommitment)
	for _, inJ := range publicCommitted {
		inJ.FillBytes(res[offset : offset+fieldByteLen])
		offset += fieldByteLen
	}

	return res
}

[ivokub]

Looks good to me, pinging @Tabaie to confirm.

[Tabaie]

LGTM!

[ivokub]

Thanks for the contribution!