src: introduce new constructors

Signed-off-by: Jean Privat <jean@pryen.org>
src: new annotation old_style_init

Signed-off-by: Jean Privat <jean@pryen.org>
modelize_property: simple inits are new-style

Signed-off-by: Jean Privat <jean@pryen.org>
tests: update to pass tests

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add base_init_basic.nit

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