Lowers MapSet functions on a HashSet or BTreeSet for RustQ.Meta.Lower.Stdlib, and
types such calls: new/0 (new/1,2 are comprehensions; see RustQ.Meta.Desugar), put, delete, member?, union, difference, intersection,
subset?, disjoint?, size, and to_list. An update takes the set only at its last use;
see RustQ.Meta.Lower.Collection.