Add ExponentialBackoff

[pior]

ExponentialBackoff(base, maxDelay) returns a backoff for ErrorBackoff: base, 2·base, 4·base, … capped at maxDelay. No jitter.

runnable.Retry(job, runnable.ErrorBackoff(runnable.ExponentialBackoff(time.Second, time.Minute)))

• Doubling stops at the cap, so a large error count cannot overflow.
• If maxDelay is below base, every delay is maxDelay.
• The default backoff is unchanged.