Summary:
`Sequence` API to walk over free variables in expressions, instead of computing lists with uniqueness constraints that make them have linear complexity for insertion.
Switch to a Set representation when we don't care about the order of elements,
otherwise to a `Hash_queue`:
https://ocaml.janestreet.com/ocaml-core/113.33/doc/core/Std/Hash_queue.mod/S.modt/
Often, we don't even need to compute the sequence of free variables, as we are
just testing membership/emptiness/...
Reviewed By: mbouaziz
Differential Revision: D7099294
fbshipit-source-id: e96f84b
Jules Villard
bc7a71d413