Introduce AbstractModelStatement
Since we now have abstract classes for both declared and effective
worlds, it is useful tie the implementations together, so that it
both share vtable where ModelStatement methods are concerned.
Also unify substatement list handling between the two implementations,
sharing the utility methods to do so.
JIRA: YANGTOOLS-1065
Change-Id: I52849b2afa6a20478a980ba0992dbcc338941dd9
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit
713fdd926360af53c70c871f441be23ef0927889)