*   `isoVar`, `isoVar2`, `isoVar3`, and `isoVarN`: convenient aliases for
    applying isomorphisms to `BVar`s.  Helpful for use with constructors and
    deconstructors.
*   `opIso2` and `opIso3` added to *Numeric.Backprop.Op*, for convenience.
*   `T0` (Unit with numeric instances) added to *Numeric.Backprop.Tuple".

*Internal*

*   Completely decoupled the internal implementation from `Num`, which showed
    some performance benefits.  Mostly just to make the code slightly cleaner,
    and to prepare for some day potentially decoupling the external API from
    `Num` as well.