Define explicit to_xml() in Token, ParseTreeNode.

10_compiler_i_syntax_analysis/jackcompiler/syntax/(parser, tokenizer).py
	-Rename `Token` and `ParseTreeNode`'s current `__str__()` to
	more explicit `to_xml()` methods, since XML isn't always (read:
	like, ever) the most desirable/human-readable representation of
	the objects. Leave `__str__()` open to overriding.