protobuf/upb/google

This directory contains code to interoperate with Google's official
Protocol Buffers release.  Since it doesn't really have a name
besides "protobuf," calling this directory "google" seems like the
least confusing option.

We support writing into protobuf's generated classes (and hopefully
reading too, before long).  We support both the open source protobuf
release and the Google-internal version of the same code.  The two
live in different namespaces, and the internal version supports some
features that are not supported in the open-source release.  Also, the
internal version includes the legacy "proto1" classes which we must
support; thankfully this is mostly relegated to its own separate file.

Our functionality requires the full google::protobuf::Message
interface; we rely on reflection so we know what fields to read/write
and where to put them, so we can't support MessageLite.