Retry with Exponential Backoff

Implement retry(fn, retries, baseDelay) for an async function fn that may fail.

It calls fn() and returns a promise that:

• resolves with the result of the first call that succeeds,
• after a failed call, waits and calls fn again, up to retries extra times (so fn runs at most retries + 1 times),
• waits baseDelay * 2^k milliseconds before retry number k + 1 (exponential backoff: baseDelay, then 2 * baseDelay, then 4 * baseDelay, ...),
• if every attempt fails, rejects with the error from the last attempt.

There is no delay before the first call and none after the last failure. retries may be 0.