Follow-up from "Optimize binary decoder for unions"

The following discussion from !24 (merged) should be addressed:

• @raphael-proust started a discussion:

  When we build the union originally, there's sharing between the array and the list: elements of the lists that have the Tag tag are physically equal to one element of the array.

  After lifting, there is no sharing. I don't know if this is a problem, it shouldn't take too much space, and for the cache it should also be ok because for most encodings we use either the json or the binary encoding, but rarely both repeatedly.

  ---

  If we want to preserve sharing, we can do something along the lines of:

  let tagged_cases = Array.copy tagged_cases in
  let cases =
    List.map
      (fun case ->
        let case = lift case in
        if is_tagged case then
          update_array tagged_cases case;
        case)
  in
  …

  ---

  (Note: the same comment applies to other occurrences of this pattern of code.)