Middleware Pipeline

Implement createPipeline(), an Express/Koa-style middleware pipeline. It returns an object with:

• use(middleware) appends a middleware and returns the pipeline (so calls chain).
• run(ctx) runs the middleware in order and returns a promise that resolves with ctx when the whole chain has finished.

A middleware is (ctx, next) => any and may be async. Calling next() runs the following middleware and returns a promise that resolves when it (and everything after it) has finished, so code after await next() runs on the way back. Rules:

• If a middleware does not call next(), later middleware do not run.
• After the last middleware, next() simply resolves.
• If a middleware throws or rejects, run rejects with that error.
• Calling next() more than once from the same middleware rejects with an Error whose message contains "next() called multiple times".
• The pipeline can be run many times; each run is independent.