Remove positions map from the tree structure

[ureeves]

Summary

Given the fact that we can arbitrarily walk the tree, it shouldn't be necessary to include existing positions since we could use a walker to determine both how many positions in the tree are filled (Tree::len()), as well which positions are filled (Tree::contains).

Relevant Context

Merkle trees sometimes carry information about the pre-image of their leaves (data blocks). Since a decision was made to have this tree not contain this, and the position field feels like much the same information, it should also be removed.

Possible Solution (optional)

Use a pre-defined walk to iterate through all leaves by counting them.