Eliminate the concept of imbuing messages in favor of the simpler concept of setting a cap table directly on MessageReader / getting one from MessageBuilder.  This eliminates capability-context entirely.  This was made possible by the earlier change which moved capabilities to a separate table rather than storing CapDescriptors inline, but I didn't realize it at the time.

Hack to make it safe to read capabilities from default values (returning broken caps) without introducing a dependency from libcapnp on libcapnp-rpc.

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.

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.

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.

Mark every destructor "noexcept(false)" unless it is very clearly noexcept.  The change in C++11 making destructors default-noexcept was a mistake.

totalSizeInWords() method on generated struct readers/builders returns total size of the struct, not counting far pointer overhead.  Useful for allocating space for a flat copy.  Also a couple misc tweaks.

Starting on dynamic message manipulation (manipluating messages without knowing the schema, or without compiling in the schema).

Get rid of redundant ErrorReporter that goes in ReaderOptions.  Registering an ExceptionCallback is a better approach.

Rename the 'Message' interfaces to Arena and make them internal.  Make a new, intuitive 'Message' interface for creating / reading messages.

Moar type-safety, rewriting WireFormat to avoid descriptors (incomplete), decided struct pointers should point at the beginning of the struct after all.