The few new/delete/free functions from the dwg_api are basically useless,
as they don't add/delete the entity to the DWG. We need to add a Dwg_Object
and the associated Dwg_Object_Entity and then the Dwg_Entity_CIRCLE.
Same for delete.

See encode.c for dwg_add_##ENTITY for the proper API, but these are untested (waiting for dwgwrite/dxf2dwg).

Use dwg_add_##ENTITY and dwg_free_object() instead.

Add these to the header, and document it.