Make the Timber class statelessly update siblingPaths

[IlyasRidhuan]

The timber class should be able to do the following:

(leafIndex, siblingPath, currentFrontier, currentLeafCount) x incomingLeaves -> updatedSiblingPath.

Where the updatedSiblingPath is the valid path for leafIndex in the context of a Timber instance with incomingLeaves added.

It should do this statelessly, i.e. without requiring the current tree structure.