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