The DocumentStructureEnumerator class added as part of SERVER-29795 always uses 0 for the scalar value in documents it generates. The UpdateSequenceGenerator class introduced in SERVER-29939 randomly selects between null, true, false, and double values between -INT_MAX and INT_MAX. In order to be able to detect idempotency issues related to different BSON types (e.g. type fidelity in $set, see SERVER-16801), we should accomodate being able to generate all possible BSON types.

To work around the lack of a BSONValue class (i.e. a BSONElement is a view into the memory owned by a BSONObj), we should use the Value class from the aggregation framework as the generic return type of some kind of generate() method.

We should have two concrete implementations of the ScalarGenerator interface:

1. TrivialScalarGenerator should just return Value(0) and is what the self-tests for DocumentStructureEnumerator should use.
2. RandomizedScalarGenerator should implement the existing behavior of UpdateSequenceGenerator.