Merge readPos, grabPos and descendToPos

[dergoegge]

Currently there are three two separate functions to retrieve a node from the pollard. It would be nice to just have one function that combines their functionality.

readPos gets the node at the position if possible but does not mutate the pollard.
grabPos gets the node at the position if possible and populates the siblings along the path that don't exist.
descendToPos returns the path to the target node, as well as the sibling path.