Avoid quadratic time in subsumed elements when adding a super-type of
existing union elements.
Reserve space in advance when adding multiple elements (from another
union) to a union.
Make union elements a `Box<[Type]>` instead of an `FxOrderSet`; the set
doesn't buy much since the rules of union uniqueness are defined in
terms of supertype/subtype, not in terms of simple type identity.
Move sealed-boolean handling out of a separate `UnionBuilder::simplify`
method and into `UnionBuilder::add`; now that `add` is iterating
existing elements anyway, this is more efficient.
Remove `UnionType::contains`, since it's now `O(n)` and we shouldn't
really need it, generally we care about subtype/supertype, not type
identity. (Right now it's used for `Type::Unbound`, which shouldn't even
be a type.)
Add support for `is_subtype_of` for the `object` type.
Addresses comments on https://github.com/astral-sh/ruff/pull/13401
149fb2090e