Transform tokens + Minor bugfixes

Lark transformers can now visit tokens as well. Use like this:

class MyTransformer(Transformer):
    def TOKEN1(self, tok):
        return tok.upper()

    def rule_as_usual(self, children):
        return children

MyTransformer(visit_tokens=True).transform(tree)

Fixed a few regressions that I accidentally added to 0.7.4