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

[ImmemorConsultrixContrarie]

This implementation is unsound:

legion/src/internals/storage/packed.rs

Lines 421 to 432 in 0d0a539

	/// Moves a component to a new storage.
	fn transfer_component(
	&mut self,
	src_archetype: ArchetypeIndex,
	src_component: ComponentIndex,
	dst_archetype: ArchetypeIndex,
	dst: &mut dyn UnknownComponentStorage,
	) {
	let component = self.swap_remove_internal(src_archetype, src_component);
	unsafe { dst.extend_memcopy_raw(dst_archetype, &component as *const T as *const u8, 1) };
	std::mem::forget(component);
	}

because this compiles:

fn soundness_hole() {
    let mut a: PackedStorage<u64> = Default::default();
    let mut b: PackedStorage<u8> = 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.