Address referee feedback by adding support for serializing function,
language, and environment objects with serialize_pb.  It's still not
particularly useful since these are mostly R language constructs, but
still I agree it will make our exposition clearer in section 6 and
makes this functionality feel more complete.

Add a unit test verifying that all 106 built-in datasets in R can be
round-trip serialized/unserialized into protocol buffers without
error.



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rprotobuf@913 edb9625f-4e0d-4859-8d74-9fd3b1da38cb