Change serialization format to include type field.

All objects serialized derive now from Osmium::Ser::Item and contain their own
size plus a type field. Currently the type field is not set for every object
type, but after we have added this, reading different kinds of objects can
be made more flexible. The resulting dump file is already smaller, because
we only use 32 bit sizes for objects (which is plenty) instead of 64 bit.