Miscellaneous things to do, in roughly descending order of priority.

- Automatic tests for encoder/decoder.
  => Have descriptor.proto, need much more.
  => Bytes.
  => Float/double.
  => Every integer type.
  => Permute fields in message.
- Proplib compiler.
- Automatic tests for proplib encoder/decoder.
- Unions, or `oneof's.
- Groups.
- Text format.
  => Move test/dumpdesc/pb_dump.c to a libpicopbtext, perhaps.
  => If we want to parse in a library, need a better parser generator
     than yacc -- consider lemon.  (Need destructors for objects on
     stack in case of error.)
