Merge block_token._root_node and span_token._root_node into a single object

[anderskaplan]

This is a proposed refactoring to clean up the state management during parsing a tiny little bit. Instead of having separate _root_node references in the block_token and span_token modules, we merge the two into one and move it to a module already referenced by both of them: token.py.

[pbodnar]

Nice DRYing, thank you. :)