typing: refuse a null value in superstrings

Signed-off-by: Jean Privat <jean@pryen.org>
tests: test new policy in test_superstring

Signed-off-by: Jean Privat <jean@pryen.org>
update code to comply with new superstring policy

Signed-off-by: Jean Privat <jean@pryen.org>
lib/standard: Added is_upper and is_lower to string.nit

Signed-off-by: Frédéric Vachon <fredvac@Pratchett.(none)>
lib/standard: Added to_snake_case and to_camel_case to string.nit

Signed-off-by: Frédéric Vachon <fredvac@Pratchett.(none)>