Use set of literals in Relation API

[bclement-ocp]

I believe equalities found here ultimately end up in Ccx.make_unique which does deduplicate. I think usually we won't find too much duplicates here (at most the number of constraints anyways) so that it shouldn't be an issue; I'd rather update the Relation API to use a set everywhere rather than do it locally. I will open an issue for this.

Originally posted by @bclement-ocp in #1056 (comment)