That way, we don't need to know the specific type of iterator or even
iteree anymore. This can turn out useful when working with `Box<dyn _>`,
that is, in contexts where we don't know the (possible or actual) types
at compile time.
This declutters the main `xso` namespace. In addition, if (when) we
introduce more complex generic implementations, we might want to have
tests for these, and those can then live there, too, without making the
main `lib.rs` file gigantic (or moving the tests too far away from the
tested code).
