miny: `dyn Any` helpers don't actually work

[1e1001]

I'm not quite sure what the issue is (and I don't really have a good reproduction yet), but the downcast_* functions don't really work for a Miny<dyn Trait> where Trait: Any, and possibly more cases.

[1e1001]

Possibly related to this warning on the documentation of Any. Maybe the T: ?Unzied + Any bound makes it call <dyn Trait as Any>::type_id() (equivalent to TypeId::of::<dyn Trait>()) isntead of <dyn Trait as dyn Any>::type_id()?

Given the existence of Miny::unsize() it should be possible to unsize to a dyn Any for the box-style downcast, and just removing downcast_mut / downcast_ref, to instead use them on a dereferenced &mut dyn Any.