Remove warnings / mark where some work is needed.
Add position argument to buffer constructor.

This way we can tell the buffer which part of it is already initialized and
which part only contains garbage. This now also fixes the
serialization/deserialization test.
Fix simple dump output and add to osmium_serdump.

osmium_serdump can now call the debug handler or just a simple dump output of
the deserializer that shows the type and size of objects in a file. Use the
--dump option to osmium_serdump if you want this.