This module exports a
Structued type class which can be used to
convert Haskel "record types" to
BSON objects and vice versa.
As a Mongo document has an "_id" field, we impose the requirement
a record type have a field whose type is
SObjId (corresponding to
Structured class used to convert between a Haskell record type and BSON document.
Type corresponding to the "_id" field of a document in a structured object.