ChainRules types in FiniteDifferences to replace to_vec

As suggested in https://github.com/JuliaDiff/FiniteDifferences.jl/issues/90#issuecomment-650078638 , we might want to consider moving away from to_vec towards defining operations on ChainRules's types directly.

In particular #91 implements the difference operation, which is the only operation that we're missing to let us approximately compute tangents. Still TODO is

1. determine how to compute cotangents within this framework + implement it, and
2. determine whether or not we require some notion of a Jacobian (probably we do) and implement it, and
3. define the norm of a differential, from which we get isapprox for free if we also define subtraction, which is really easy because we've already defined addition and scalar multiplication. I think we can do this because I think we can always derive an appropriate norm for a differential, that is, I think we can treat any given differential as being an element of an appropriate normed vector space. (Note that we've not defined inner products between differentials, and I don't think we need to. Doing so would be one way to go about defining a norm, but it probably makes sense to go straight to a norm if we can't think of a reason why would need to define inner products. I'm open to suggestions here.) This is something we'll need to sort out in ChainRulesCore, as per https://github.com/JuliaDiff/ChainRulesCore.jl/issues/184.

Implementing this will immediately resolve:

1. #92
2. https://github.com/JuliaDiff/ChainRulesTestUtils.jl/issues/24 as it is covered by difference already

but will generally significantly improve the compatibility between ChainRuless and FiniteDifferences, and should make one's experience testing differentials a much more pleasant experience.

Start by reviewing the ChainRules types and the difference operation from #91, then read ChainRulesCore issue #184 for the unresolved norm and subtraction design. Determine how cotangents and Jacobians should work, implement the missing operations, and verify that #92 and ChainRulesTestUtils issue #24 are resolved.