fix: various issues with diffing algorithm

This commit reimplements the way we track changes across two documents.
Previously cursors in a document during a diff solely consisted of
parent node, depth in the tree, and child index under the parent. While
this was sufficient for some trivial cases, a number of real world
scenarios fall apart with this due to a lack of information needed to
correctly identify where in the tree a change occurred. The new cursors
track the full path from the root of the document to the current cursor
position, which allows us to correctly determine where in the tree a
change occurs in some of the cases where previously we could not.

It remains to be seen if there are more holes in our implementation, but
for now our current set of test cases are correctly diffed/merged with
the new algorithm.

Closes #3

Author: bitwalker