Refactoring: Move osmium::area::NodeRefSegment into detail namespace.
Refactoring: Assembler class now instantiated for every way/mp relation.

The Assembler class used to instantiated once and its data members cleared
before each way or multipolygon relation was given to it. This is rather
strange and the new solution is much clearer. To hold the configuration
needed for the Assembler class a new struct AssemblerConfig was introduced.
Allow setting debug flag in AssemblerConfig constructor.
Clean up code comments.
Updates examples to use new area::Collector/Assembler interface.
Add chain handler.
Add callback to collector handler.

The callback is called with a buffer containing collected areas.
Rename ogr_handler to test_handler.
Use new callback syntax in osmium_mpdump
Split storage buffers for relations and members in relations collector.
Improve formatting of statistics output in relations collector.