ArrayValidator: add subpointer calculation method plus tests

This is one place where the schema path is important.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>
ObjectValidator: add a static, package visible class for a pointer + node

We have to do that. Unlike with ArrayValidator where only one schema is returned
per child and we can calculate the pointer separately, here we must bundle the
pointer plus the subschema in one object.

Signed-off-by: Francis Galiegue <fgaliegue@gmail.com>