* Convert BlockGHOSTDAGData from an interface to a public struct with getters
* Move hashes.Less to externalapi so it can access the hashes directly without copying
* Reduce calls to ghostdagstore.Get in blueWindow
* Simplify the logic in RequiredDifficulty and reuse big.Int
* Remove bigintpool as its no longer used
* Use ChooseSelectedParent in RequiredDifficulty instead of looping over the parents
* Remove comment