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.

Allow implicit conversion from Orphan<T> to Orphan<DynamicStruct/DynamicList/DynamicCapability> where applicable.

Fix bug handling absolute source paths with no --src-prefix -- just output to the same directory in this case.

Define new KJ constants for min/max integer values and inf/nan, then remove all use of numeric_limits.

Spin off libraries kj-async and capnp-rpc, so that people who don't want them can avoid their large footprint.