Make `diverging_type_vars` a vec of `TyVid` r? @lcnr The following changes, in separate commits: * Make its elements a `TyVid`, since there should never by any other types than `TyVars` in there * Make it a vec, since it being a set doesn't make much sense. You never really should do a `contains` on it, since you should normalize the tyvids in the set to their root var first.