Ptr container:
  MarshalBSON()  'listed'->1
  MarshalBSON()  'mapped'->2
  UnmarshalBSON()  text listed|1
  UnmarshalBSON()  text mapped|2
! ptr == ptr2
  ptr1: {['listed'->1] map[ptrKey:'mapped'->2]}
  ptr2: {['listed'->1] map[ptrKey:'mapped'->2]}
Obj container:
  MarshalBSON()  'listed'->1
  UnmarshalBSON()  text listed|1
  UnmarshalBSON()  
Error unmarshaling objContainer: error decoding key index.objKey: wrong number of pieces: 1
* obj1 != obj2
  obj1: {[{listed 1}] map[objKey:{mapped 2}]}
  obj2: {[{listed 1}] map[]}