[topology] have Neighbor/Child/Router inherit from InstanceLocatorInit (#4569)

This commit changes the `Neighbor` class (and therefore its sub-classes
`Child` and `Router`) to inherit from `InstanceLoatorInit`. It adds
`Init()` method to them as well and ensures that they are initialized
before use from `ChildTable`, `RouterTable` and `Mle` classes.

tests/unit/test_child.cpp

@@ -120,6 +120,8 @@ void TestChildIp6Address(void)
     sInstance = testInitInstance();
     VerifyOrQuit(sInstance != NULL, "Null instance");
 
+    child.Init(*sInstance);
+
     //- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
     printf("\nConverting IPv6 addresses from string");