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.

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