There is significant refactoring here, as well as some more trivial
name changes. upb_msg has become upb_msgdef, to reflect the fact
that a upb_msg is not *itself* a message, it describes a message.
There are other renamings, such as upb_parse_state -> upb_stream_parser.
More significantly, the upb_msg class and parser have been refactored
to reflect my recent realization about how memory management should
work. upb_msg now has no memory management, and a memory mangement
scheme (that works beautifully with multiple language runtimes) will
be layered on top of it.
This iteration has the new, read-only upb_msg. upb_mm_msg (a
memory-managed message class) will come in the next change.