Redefine ParseTreeNode and Token as classes.

10_compiler_i_syntax_analysis/jackcompiler/syntax/(parser, tokenizer).py
	-Redefine `ParseTreeNode` and `Token`, previously `namedtuple`s,
	as classes, to allow custom `__str__()`. This allows us to
	remove `to_xml()`, and simply rely on recursive `__str__()`
	calls.