This reverts commit dc2da0edc5.
Add pb_close_string_substream() for copying back the state.
This makes adding error messages easier in the future, as also
them need to be propagated back from the substream.
This makes the field decoding functions more intuitive to use.
The old interface is still present if you specify NANOPB_INTERNALS.
Update issue 2
Status: FixedInGit