nitcc: add Empty keyword

Signed-off-by: Jean Privat <jean@pryen.org>
nitcc: classes and elements allow any forms of text

Signed-off-by: Jean Privat <jean@pryen.org>
nitcc: add Token::build_nfa to isolate concerns

Signed-off-by: Jean Privat <jean@pryen.org>
nitcc: avoid conflict in ignored for test/empty

Signed-off-by: Jean Privat <jean@pryen.org>
nitcc: allows elem_list in Ignored

Signed-off-by: Jean Privat <jean@pryen.org>
nitcc: add test/ignored

Signed-off-by: Jean Privat <jean@pryen.org>