Add a test where a locked pointer shall be taken during object destruction.
Moreover, extend the run-script so it runs on different platforms with
"real" timers.
So far, the lifetime-management utilities 'Weak_ptr' and 'Locked_ptr'
had been preserved for core-internal use only. However, the utilities
are handy for many use cases outside of core where object lifetimes
must be managed. So we promote them to the public API.
