Row_like indexes allows to represent a range rather than only upward closed set

[chambart]

This allows to more precisely represent the size of joins of polymorphic variants. This would be useful to be able to unbox something like:

let f b x =
  let v =
    if b then
      `A (x, x)
    else
      `B (x, 1, 2)
  in
  match v with
  | `A (a, b) -> a + b
  | `B (a, b, c) -> a + b + c

Notice that this is not enough to allow simplification of c to 2 here. Join still considers that joining between a known case and an absent case is equivalent to joining with top, where it should be equivalent to joining with bottom. We would need to refactor a bit to do that, which I defer for later.

In a first version the Range constructor took an option for the at_most field rather than having an At_least and Range case. But there is no factorisation to gain that way and the code is a bit cleaner like that.

[Gbury]

How can an index be None, from what I recall, Indexes can be integers at least in some cases...
Also, it's a bit weird to have this redundancy in how to represent at_least (i.e. either directly with At_least, or with Range using a None at_most), but no way to represent just the at_most case.

[chambart]

Oups I didn't update the comment. The case was

| Range of { at_least : Index.t; at_most : Index.t option; }