Change capability pointers to be indexes into a separate cap list so that CapDescriptors can be interpreted on receipt rather than delaying until the application actually traverses the message.  This massively simplifies a lot of things.

Get rid of 'Advanced' server methods, just have one method that takes a CallContext.  A typedef reduces boilerplate.

Make all Promise methods consistently consume the promise (returning a new promise when it makes sense), rename daemonize -> detach, and make eagerlyEvaluate() require an error handler (this caught several places where I forgot to use one).

Split AsyncIoProvider into high-level and low-level interfaces, so that alternate EventPort implementations can implement the low-level interface and reuse the higher-level stuff.

Overhaul the way EventLoop is specialized so that it's possible to hook up to an existing event loop infrastructure that is not KJ-aware.  This also makes the async IO API more dependency-injection-friendly.

Eliminate the ability to have multiple threads working on building the same message -- performance penalty is too large, and applies even to single-threaded users.

Revamp concurrency model, part 1:  EventLoop no longer allows cross-thread event queuing, simplifying many things.  Capability clients are no longer thread-safe, so they don't have to be so const.  In the future, explicit ways to communicate between threads will be re-added, but threads will be treated more like separate vats that just happen to have a particularly fat pipe.  Upcoming:  Remove mutexes.

Make EventLoop current when constructed, so that you can use the Promise methods even when the loop hasn't actually started yet.

Implement opt-in cancellation for local calls.  (Previously, cancellation would happen whether or not the callee had opted in.)