Add checkpoints to staking contract

[cygnusv]

Adds checkpoints to staking contract, so that stakers can collect and delegate voting power in proportion to their stake size.
Some important aspects to note:

• Delegation of stake voting power is controlled by the owner (i.e., the owner decides to whom delegate their vote)
• Threshold model of staking roles allows for different stakes, associated to different operators, but with the same stake owner. Conversely, this means that the same owner address can have multiple stakes associated to different operators. In practice, in this situation the owner must delegate each stake individually.
• Because of the particularities of the Threshold model of staking roles, the delegation function requires an extra parameter, that is, the operator address, in order to uniquely identify each stake:

    /// @notice Delegate voting power from the stake associated to the
    ///         `operator` to a `delegatee` address. Caller must be the owner
    ///         of this stake.
    function delegateVoting(address operator, address delegatee) public {
        delegate(operator, delegatee);
    }

[vzotova]

I guess there is missing commit with new optimizer runs or something causing the TokenStaking deployment to fail during tests

[pdyraga]

I guess there is missing commit with new optimizer runs or something causing the TokenStaking deployment to fail during tests

Have we not just exceeded 24KB contract size limit with the additions from this PR?

[cygnusv]

I guess there is missing commit with new optimizer runs or something causing the TokenStaking deployment to fail during tests

Have we not just exceeded 24KB contract size limit with the additions from this PR?

Yeah, but reducing the number of optimization rounds allows to be right under 24KB

[pdyraga]

There is some infrastructure-level problem with the draft system tests job. Given system tests are still in draft, I am ignoring it and merging.

[pdyraga]

Nitpick but we could keep the same order of operations of emitting event, updating staked amount, and decreasing checkpoint in unstake, unstakeKeep, and unstakeNu.

[pdyraga]

What if we rename delegate function in T token contract to delegateVoting to keep them aligned?

solidity-contracts/contracts/token/T.sol

Line 79 in fa83b98

function delegate(address delegatee) public virtual {

We could also rename the function in Checkpoint so that we do not need to have function delegate(address operator, address delegatee) a few lines below and just put this logic into delegateVoting.

[pdyraga]

I think we should add a test that it should decrease the total voting power as well.