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.