`<PackedStorage<T> as UnknownComponentStorage>::transfer_component` is unsound

This implementation is unsound:

https://github.com/amethyst/legion/blob/0d0a53953465ca73e6f9231ecbae9044bf4b4389/src/internals/storage/packed.rs#L421-L432

because this compiles:

```rust
fn soundness_hole() {
let mut a: PackedStorage = Default::default();
let mut b: PackedStorage = Default::default();
let a: &mut dyn UnknownComponentStorage = &mut a;
a.transfer_component(ArchetypeIndex(0), ComponentIndex(0), ArchetypeIndex(0), &mut b);
}
```

Getting this UB outside of lib is tricky, however unsoundness inside the lib itself is bad too.