Build a circuit breaker library

• new CircuitBreaker(fn, options) wraps an async function and exposes fire(...args), which calls it and returns or rethrows its result
• States: closed (calls pass through), open (calls fail immediately with a CircuitOpenError without calling the function) and half-open (a limited number of trial calls are allowed)
• Closed to open when failures reach a threshold: a count of consecutive failures, or a failure percentage over a rolling window with a minimum number of calls
• Open to half-open after resetTimeoutMs; in half-open, a successful trial closes the circuit and any failure re-opens it and restarts the timer
• Optional fallback(...args) runs when the call fails or the circuit is open, and an optional per-call timeout counts as a failure
• Metrics are available (successes, failures, rejected calls, timeouts, current state) and an event emitter reports state changes (open, halfOpen, close)

Only after every requirement works.

• Distinguish errors that count as failures from ones that do not (for example 4xx)
• Bulkhead (maximum concurrent calls)
• Sliding-window statistics with buckets