optimize Quorum

[ZzzzHui]

📚 Context

This issue is to optimize the PR to add quorum, which addresses this issue.

• We can optimize the Quorum by getting rid of AccessControlEnumerable and EnumerableSet.
• Furthermore, instead of using mapping(address => bool) voted; to note who has voted, we could consider splitting a uint256 into bits and utilize bit position to denote whether a validator has voted.

For details, please see this comment

✔️ Solution

Since the last part (split a uint256) of the optimization can be complex and we are not sure whether we will go for that route, we will open 2 PR for this issue.

[ZzzzHui]

Splitting a uint256 into | who_has_voted(248 bit) | count(8 bit) | is the most optimal. But it can be error prone as well. So I wanna take the middle ground by using a 256 bit map to indicate whether a validator has voted, and a uint256 var to count the number of votes.

The openzeppelin library called BitMaps is most suitable for this case. Compared with the current solution (mapping(address => bool) voted), which uses 1 storage slot for each validator, BitMaps use only storage slot for every 256 validators (or 255 as we don't use the bit on index 0 for the first slot).

Compared with the current solution, BitMaps saves on gas because write a non-zero storage slot (2,900 gas) is much cheaper than writing a zero storage slot (20,000 gas), excluding the access operation gas.

BitMaps solution is on branch feature/optimize-quorum-bitmap
PR opened here

P.S. OpenZeppelin BitMaps is basically the same as our library BitMask