** Implement to convert custom Fantom types to / from a BSON representation.
const mixin BsonConv {
** Converts a Fantom object to its BSON representation.
**
** Must return a valid BSON value (or a List or Map thereof).
**
** 'fantomObj' is nullable so converters can create empty / default objects.
abstract Obj? toBsonVal(Obj? fantomObj, BsonConvCtx ctx)
** Converts a BSON value to Fantom.
**
** 'bsonVal' is nullable so converters can create empty / default objects.
abstract Obj? fromBsonVal(Obj? bsonVal, BsonConvCtx ctx)
}