Mark inline assembly as memory-safe

[davidlaprade]

Fixes #52

The GovernorCountingFractional contract will be removed in a future release when we bump OZ to v5.1. But for the sake of users that can't upgrade to OZ v5.1, we're marking the use of assembly in that contract as memory-safe.

Per the solidity docs:

The only assembly we have is here:

    function _decodePackedVotes(bytes memory voteData)
        internal
        pure
        returns (
            uint128 againstVotes,
            uint128 forVotes,
            uint128 abstainVotes
        )
    {
        assembly {
            againstVotes := shr(128, mload(add(voteData, 0x20)))
            forVotes := and(_MASK_HALF_WORD_RIGHT, mload(add(voteData, 0x20)))
            abstainVotes := shr(128, mload(add(voteData, 0x40)))
        }
    }

This assembly doesn't write to any memory variables. Nor does it allocate any. Instead, it assigns to return variables which solidity has already assigned to stack slots. It only reads from the voteData memory param, and does so within its already-allocated bounds (the caller would have reverted if voteData.length != 48). It doesn't modify any scratch space or the free memory pointer.

[github-actions]

Coverage after merging memory-safety-fixes-#52 into master will be

91.67%

Coverage Report

File	Stmts	Branches	Funcs	Lines	Uncovered Lines
src
FlexVotingClient.sol	100%	100%	100%	100%
FractionalPool.sol	79.63%	70.59%	73.33%	82.89%	107, 109, 111, 114, 190–191, 193–195, 198, 202, 238–239, 78, 80, 82–83, 85
GovernorCountingFractional.sol	94.59%	76.92%	100%	98.04%	184, 186, 188, 191