command results. The names of the types match the names of the
MongoDB server commands. If we just want a single value out of the result,
then it is probably more efficient to use the MongoCommon.dotresult_type functions.
If we want to manipulate the whole thing in OPA, however, we can just
Bosn.doc2opa the result and cast to these types.
Note that these types
may vary between one MongoDB version and another, these are for 1.8.3. Note also that some of the results have floating elements which we
map to the Bson.register type.