emoji_eventsRate limiter (sliding window)

Implement a RateLimiter(limit, window) class whose allow(key, now) method returns True if the request is allowed.

• At most limit requests per key in any window seconds (a sliding window, not fixed buckets).
• now is passed in (seconds as a float) so the tests are deterministic.
• Different keys are independent.
• Memory for a key should not grow forever: drop timestamps that left the window.