I am not sure if removing initializers from `SetAlgebra` does not cause any
problems in the standard library, it seems reasonable for me if range types
could adopt `SetAlgebra`.
`SetAlgebra` contains some mutating methods like `insert`, `remove`,
`formUnion`. It means it is impossible for range types to adopt
`SetAlgebra`. In addition, even if we remove such methods from
`SetAlgebra`, because the return type of something like `union` is `Self`,
`2..3.union(5...7)` cannot be represented.
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