lib: add combinations.nit for combinatoric collections

Signed-off-by: Jean Privat <jean@pryen.org>
lib: add cartesian.nit a libraby for heterogenus cartesian products

Signed-off-by: Jean Privat <jean@pryen.org>
tests: update because line changes in lib

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