[list-ops]: change // to / and swap foldl/foldr arg order to both call func(el, acc)

[IsaacG]

No description provided.

[BethanyG]

@IsaacG - If we are going to break existing solutions, I would vastly prefer to have foldr & foldl both be (acc, el), so that foldl is consistent with Haskell tradition. But I am going to go with your first PR, so we can be done with this debate. 😄