Add Condition->trySucceed and Condition->tryFail (#185)

[Atry]

Summary:
It would allow for use cases like this:

async function outer(): Awaitable<void> {
  await wait_for_notification_async(
    async $notifyee ==> {
      concurrent {
        await async {
          await gen_usleep(100);
          $notifyee->trySucceed("fast condition");
        }
        await async {
          await gen_usleep(10000000000);
          $notifyee->trySucceed("slow condition");
        }
      }
    }
  );
}

X-link: hhvm/hsl#185

Reviewed By: fredemmott

Differential Revision: D34903436

Pulled By: Atry

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D34903436

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