Tail-recursive List.map alternative.

This map function is the one described in https://discuss.ocaml.org/t/a-new-list-map-that-is-both-stack-safe-and-fast/865

CAVEAT: The order of application of the function to the elements of the list is different from that of Stdlib.List.map. The function is applied in reverse order of the elements in the list. In other words:

map_pure f l is equivalent to List.rev_map f (List.rev l)

Considering this caveat, it is recommended to use this mapping function with a pure function as argument. This is why the name map_pure is chosen.

append is a tail-rec variant of List.append. It special cases small lists, and then it switches to List.rev_append.