threads::shared tidy up:

threads::shared tidy up:
 - Add _id() function to shared.xs so we can test refs point at same thing.
 - Use that rather that comparing stringified ref in shared/t/hv_refs.t
 - Allow no_share to pass if sharing to works despite mis-ordering.
 - Change docs of threads to document ->create() (as used in tests),
   and not mention "new".

p4raw-id: //depot/perlio@14410