Finish fixing data-discarding bug that colomon discovered.
Add conversion to native p6 data structures.
Rename 'JSON' to 'Jansson'.

This keeps a clearer distinction between data-exposing-interfaces
(JSON::Document, JSON::Object, JSON::Array) and the internal repr
(a pointer to a Jansson object). Also makes it obvious that
'Jansson.new' and 'Jansson.decode' return Jansson reprs, not P6
structures.