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.

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

In RPC protocol, rename request -> params, answer -> results.  Also fix up Join stuff in rpc-twoparty.capnp, because it was sort of wrong.

Remove 'has' for primitive fields.  Dynamic version always returns true.  This also means that primitives are always included in text format, even if they are default-valued.

Improve inf/nan.  This also makes GCC encode them consistently with Clang and with the existing testdata.