COMPLETEInitialization has completed and all future calls should finish immediately.
By choosing this state as the all-zero state the is_completed check can be
a bit faster on some platforms.
INCOMPLETENo initialization has run yet, and no thread is currently using the Once.
POISONEDSome thread has previously attempted to initialize the Once, but it panicked,
so the Once is now poisoned. There are no other threads currently accessing
this Once.
QUEUEDMay only be set if the state is not COMPLETE.
RUNNINGSome thread is currently attempting to run initialization. It may succeed,
so all future threads need to wait for it to finish.