Change temporary identityHashCode() feature holder id to persistent UUID

That is required because
* identityHashCode() is not guaranteed to be unique and
* the id should be persistent (not possible with identityHashCode())
Improve TreeWalker performance

* By using a HashSet of UUIDs instead of a list of feature holders for
cycle detection.
* By adding the extra "callPost" parameter for suppressing unnecessary
postVisit() calls.
Fix TreeInitializerTest and add TreeWalkerTest
Improve Default(Collection)Property accessor speed (get/set/add/remove)

That is done by only creating accessor functions if custom accessor
executors are available and therefore reducing unnecessary function
calls.