model: add Type#length

Signed-off-by: Jean Privat <jean@pryen.org>
make: compile nitg with -O

c7fa2b1c75ce1a90f86be837272d7b460a3cf775 dropped the -O for nitg

Signed-off-by: Jean Privat <jean@pryen.org>
model: most anchors can be null

This help the standard case where the type are all resolved.

Signed-off-by: Jean Privat <jean@pryen.org>
model: add MType::can_resolve_for

Also beef-up most methods that require resolution with preconditions that
rely on `can_resolve_for`

Signed-off-by: Jean Privat <jean@pryen.org>
Merge branch 'model'