Minimize custom code in `Fold`

[rtar]

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Right now Fold contains a lot of custom code and syntax to make constructing and combining Fold instances easier. It may make understanding of the whole library harder, because one will have to learn a domain specific language.

The idea is to investigate prior art and reuse as much syntax as possible for Fold. I.e. Fold is actually Kleisli[F, (S, A), S], so monad syntax could be reused from it etc. The key idea could be making a less restrictive type such as IndexedFold[F, (S1, A), S2] and then reuse functionality from Kleisli to define methods for original Fold[F, S, A].

See also similar structure here: https://github.com/notxcain/aecor/blob/master/modules/core/src/main/scala/aecor/data/Fold.scala