The `pos` and `size` functions for Nodes now return TSLength structs,
which contain lengths in both characters and bytes. This is important
for knowing the number of unicode characters in a Node.
- Node position is public. It represents the node's first character
index in the document.
- Tree offset is private. It represents the distance between the tree's
first character index and it's parent's first character index.
- Tree padding is private. It represents the amount of whitespace
(or other separator characters) immediately preceding the tree.
