Improve unions.  Members of a union are now scoped within the union, although they share ordinal space with the parent struct.

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

Replace most CAPNPROTO_ASSERTs with the new logging/error macros.  Also add a SYSCALL macro and use it.  Also add commentary.

Refuse to read messages with excessive segment counts or messages whose total size exceeds the traversal limit, before allocating any space.  This is important for security.

Update far pointer implementation to match spec.  Also adjust the spec slightly so that you can tell whether a far pointer is a double-far before you dereference it.