Consider split() for Box<DynFnOnce>

[danakj]

We lost split() with FnOnceRef: 72968c4#diff-ecdf6bec3f3f1f3c1b27e1acbc1104b1573eab15975d292679e8a4d6a2518cd5L424-L463

We could potentially be able to split a Box into two, where one of them will be called and then the other would panic if called. Since DynFnOnce is not cloneable, there's no way to split with ownership involved at the moment.

For a DynFnOnce& reference, the reference itself is copyable so there's nothing needed.

What does Rust do here for Box?