[lib] Auxiliary functions in List + fixes.

These are convenient to use `command.ml` for example.

We also fix a critical bug in the `fold_left_map` family of functions,
as witnessed by this old behavior.

```ocaml
fold_left2_map (fun c u v -> c+1,u+v) 0 [1;2;3] [1;2;3;];;
- : int * int list = (3, [6; 4; 2])
```

I have opted for a simple fix keeping the tail-recursive nature, I am
not in the mood of writing base libraries, but feel free to improve.

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