Make encoder / decoder compatible with stream subclassing

[PetteriAimonen]

Currently the encoding and decoding functions make a copy of the pb_istream_t / pb_ostream_t for the substream. This means that if your own code tries to do this:

 struct my_stream_t {
        pb_istream_t stream;
        int extra_state;
 };

It doesn't work, as the extra fields do not get along when copied.

The substream implementation could instead just change the max_size on the stream structure, without copying.