Port old combinator implementations of sum types, iteration and recursion to new lambda-calculus style

[VMatthijs]

Also, some of the array primitives we had?

[VMatthijs]

(No rush, obviously)

[tomsmeding]

As for array primitives, we had only map and foldr before. Not that this is a reason to not have more, but just for posterity.

[VMatthijs]

Right! You did all the other ones (generate, permute, backpermute, indexing) in your own branch with dynamically sized arrays.

[VMatthijs]

@tomsmeding , want to try doing this at least for the sum types, so you can check whether it has the right complexity? That would then give us at least a way of doing AD on higher order programs with the right complexity by using defunctionalization.

[tomsmeding]

Oh that's a good one! I will do so. Slightly longer timeframe than usual perhaps due to lectures that I still need to make...

[tomsmeding]

@VMatthijs Seems to work with the classic LEither, but basically untested: main...sum-types
Suggestions for good test cases?