*   Added canonical strict tuple types with `Num` instances, in the module
    *Numeric.Backprop.Tuple*.  This is meant to be a band-aid for the problem
    of orphan instances and potential mismatched tuple types.
*   Fixed bug in `collectVar` that occurs if container sizes change
*   Internal tweaks to the underlying automatic differentiation types that
    decouple backpropagation from `Num`, internally.  `Num` is now just used
    externally as a part of the API, which might someday be made optional.