Defra does not take advantage of Lens-Lens enumerable

[AndrewSisley]

At the moment Defra sandwiches enumerables between Lens enumerables to facilitate input/pipe-line management. This prevents Lens from taking advantage of some very significant performance magic that shortcuts the (de)serialization of documents between Lenses.

This could be fixed within Defra (with increased startup and memory costs), or hopefully within Lens (expanding magic code to be Concat-aware).