Implemented IEnumerable<ISpecimenBuilder>

by Fixture. Together with already implementing ISpecimenBuilder (from
IFixture), it's to thirds of the way to being a proper
ISpecimenBuilderNode. However, the ISpecimenBuilderNode.Compose method
makes little sense for Fixture, but by implementing
IEnumerable<ISpecimenBuilder> it's now possible to walk the Fixture graph.
Documented GetEnumerator overloads.
Addressed a CA warning.