Use byte instead of and in the SignatureDecoderContract

[mmv08]

@roleengineer suggested that we could use byte instead of and in the signature split function. After reviewing the "industry standards" contracts, that seems to be a good decision (e.g., https://github.com/OpenZeppelin/openzeppelin-contracts/blob/a5ed318634016a25be4000ee07044a31f363e60c/contracts/utils/cryptography/ECDSA.sol#L53-L70)

This PR makes the adjustment.

[coveralls]

Pull Request Test Coverage Report for Build 6001005315

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 92.78%

---

Totals
Change from base Build 5999462738:	0.0%
Covered Lines:	320
Relevant Lines:	340

---

💛 - Coveralls