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.)

Don't rely on default assignment operators for cap clients since they break with double virtual inheritance.

Tail calls half-implemented, but I think it's time to delete all the cancellation code before continuing because it will simplify things.

Implement exclusive promise joining:  Join two Promise<T>s to create a Promise that resolves when either input resolves.

Oops, don't need to use embargoes if the resolution is on the same machine as the promise, since the network path of calls is unchanged in that case.

remove superfluous null terminator

add missing includes

kj::runIoEventLoop() is a shortcut for setting up an EventLoop that can do I/O, without specifying a platform-specific EventLoop.