This pulls the OnceSet commit out of #3332. It moves the locking correctness logic to its own type. This also adds changes for the following review comments from #3332:

• Add experimental stub_syscalls config option #3332 (comment)

  Nit: Maybe append

  assert!(!set.insert("BAR".to_string()));

  to make sure inserting still works after attempting to insert a duplicate?

• Add experimental stub_syscalls config option #3332 (comment)

  I think this is still replicating the warn_once_then_debug behavior though, right? If so, I would add that detail to the comment. And maybe a back reference to here from warn_once_then_debug so that if that function is ever changed we know to update this code too.