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: intro String::rmdir

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
android: generate Android project in a subdirectory of .nit_compile

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
android: delete the Android project folder prior to generating code

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>