Serialize way node positions.

Handler now writes dump data to file descriptor given when it is
initialized instead of always to stdout.
When node positions are available ways serialized will contain
those node positions.
Extract "parsing" code from feed() method in deserializer
Fix osmium_serget
Make parsing of serialized data more flexible.

Instead of defining what subitems and item can have, this is now
completely automatic. The deserializer will check whats in the
file and do the right thing. At the moment this allows way nodes
with positions to be deserialized properly, but in the future
this will allow more.