Reduce function `cbrt` gas consumption

I've recognized [your cbrt function](https://github.com/Equalizer-Exchange/v1-core/blob/master/contracts/libraries/Math.sol#L23-L34) from my repository.

FYI, I have recently reduced the average gas consumption of this function, feel free to adopt [the updated implementation](https://github.com/barakman/solidity-math-utils/blob/master/project/contracts/IntegralMath.sol#L57-L71).