Leftovers when clearing storage of nested Lazy-like structures

[cmichi]

I found a bug whereby we have leftovers in storage for nested Lazy-like structs.

In the example below Vec.elems: LazyIndexMap is the offender, though e.g. our HashMap is also affected by the bug.

#[test]
fn bug() {
    ink_env::test::run_test::<ink_env::DefaultEnvironment, _>(|_| {
        // given
        let root_key = Key::from([0x42; 32]);
        let mut vec: Lazy<StorageVec<u32>> = Lazy::new(StorageVec::new());
        vec.push(13u32);
        SpreadLayout::push_spread(&vec, &mut KeyPtr::from(root_key));
        let _ = <Lazy<StorageVec<u32>> as SpreadLayout>::pull_spread(&mut KeyPtr::from(root_key));

        // when
        SpreadLayout::clear_spread(&vec, &mut KeyPtr::from(root_key));

        // then
        let storage_used = ink_env::test::get_contract_storage_used::<
            ink_env::DefaultEnvironment,
        >(&contract_id()).expect("used storage must be returned");
        assert_eq!(storage_used, 0);

        Ok(())
    }).unwrap()
}

Fails with

thread 'collections::vec::tests::bug' panicked at 'assertion failed: `(left == right)`
  left: `1`,
 right: `0`', crates/storage/src/collections/vec/tests.rs:413:9