Add free_variables_list and refactor free_vars*

[rtjoa]

(Both #1518 and #1519 attempt to fix the same problem. See the last paragraph for how the behavior of this refactor differs)

The motivation for this change is similar to #1503's: we sometimes collect all free variables in a list of types tyl by passing a fake TTuple to Ctype.free_variables, as in Ctype.free_variables (Btype.newgenty (Ttuple tyl)).

However, this is fragile to changes in tuples, and in particular, becomes messy when adding labeled tuples: the above would become Ctype.free_variables (Btype.newgenty (Ttuple (List.map (fun t -> None, t) tyl))).

This refactor also allows Ctype.closed_type to return earlier, raising Non_closed for the first free variable found rather than the last (this should be the only externally-noticeable change in behavior - if we want, we could just re-collect the entire list and raise the last instead).

[ccasin]

I discussed this with @rtjoa. Although the new version is cleaner, merging it would create maintenance burden when we rebase on upstream, so I'm going to close it for now. We may make an upstream PR that combines a few of his small improvements instead.