* Implement the diffMF, etc. functionality from Numeric.FAD

* Allow the type to vary within our RAD data type container, in the same fashion as Numeric.FAD.

    Although, while Pearlmutter and Siskind provided the functionality to permit it in derivative combinator, they provided
    no combinators to convert, for instance, a Dual tag Float to a Dual tag Double, so that extra functionality cannot
    currently be leveraged.

* Implement a reverse-mode cotangent tower.

    Can we just play back the tape in a forward-mode tower?

* Provide the ability to use reverse mode locally on FAD inputs, i.e.

    reverseCheckpoint :: (forall s. RAD s a -> RAD s a) -> FAD t a -> FAD t a