1e18 * timeSinceLastUpdated * rewardsPerToken_.rate / _totalSupply
struct RewardsPerToken {
  uint128 accumulated;                            // Accumulated rewards per token for the period, scaled up by 1e18
  uint32 lastUpdated;                             // Last time the rewards per token accumulator was updated
  uint96 rate;                                    // Wei rewarded per second among all token holders
}
