Fix minor issues in ZRC2 ->ERC20 contract

[rrw-zilliqa]

... reported by HashLock:

1. SafeMath is the default in Solidity 0.8.x
2. Replace _UINT8_MAX and _UINT256_MAX definitions with the Solidity symbolic constants.
3. _UINT8_MAX should be of type uint8
4. Make immutable variables immutable