Start splitting struct json_object into multiple sub-types, as descibed at https...

Start splitting struct json_object into multiple sub-types, as descibed at https://github.com/json-c/json-c/wiki/Proposal:-struct-json_object-split
The current changes split out _only_ json_type_object, and thus have a number of hacks
 to allow the code to continue to build and work.

Originally  mentioned in issue #535.
When complete, this will probably invalidate #552.
This is likely to cause notable conflicts in any other significant un-merged
changes, such as PR#620.