Dedupe Concurrent Async Calls

Implement dedupe(fn, keyFn) for an async function fn. It returns a wrapper that shares one in-flight call between callers with the same key.

• keyFn(...args) computes the key for a call; it defaults to (...args) => JSON.stringify(args).
• If a call with the same key is already in flight (its promise is still pending), the wrapper returns that same promise and does not call fn again.
• When the promise settles, resolved or rejected, the key is released, so the next call with that key calls fn again.
• Calls with different keys never affect each other.
• The call that actually runs fn passes its own arguments and this to it.