Make nested Lazy's clear storage properly

[cmichi]

Closes #580.

The bug is that a Lazy doesn't clear up it's inner cell for clear_spread, if the inner cell has not been loaded. This can easily happen with nested Lazy's ‒ like Lazy<Lazy<…>>.

But also we often use Lazy in storage structs like StorageHashMap<…> (with values: LazyHashMap<…> ‒ if these structs are then wrapped in a Lazy there is already a nested Lazy. SmallVec, Vec, Stash or other storage structs which have this bug if they are wrapped in a Lazy.

The way I fixed this for now will probably lead to some discussion and I'm open for other approaches.

[codecov-io]

Codecov Report

Merging #583 (2fbe824) into master (3a9301f) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #583      +/-   ##
==========================================
- Coverage   67.86%   67.78%   -0.08%     
==========================================
  Files         155      155              
  Lines        6895     6914      +19     
==========================================
+ Hits         4679     4687       +8     
- Misses       2216     2227      +11     

Impacted Files	Coverage Δ
crates/storage/src/collections/smallvec/mod.rs	96.47% <ø> (ø)
crates/storage/src/collections/smallvec/tests.rs	100.00% <ø> (ø)
crates/storage/src/collections/stash/mod.rs	96.81% <ø> (ø)
crates/storage/src/collections/stash/tests.rs	100.00% <ø> (ø)
crates/storage/src/collections/vec/mod.rs	96.77% <ø> (ø)
crates/storage/src/collections/vec/tests.rs	100.00% <ø> (ø)
crates/env/src/engine/off_chain/db/accounts.rs	86.90% <100.00%> (+1.19%)	⬆️
crates/env/src/engine/off_chain/test_api.rs	74.46% <100.00%> (+3.73%)	⬆️
crates/storage/src/collections/hashmap/tests.rs	100.00% <100.00%> (ø)
crates/storage/src/lazy/lazy_cell.rs	98.46% <100.00%> (+0.15%)	⬆️
... and 3 more

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[Robbepop]

Good catch!

The fixed SpreadLayout::clear_spread impl for LazyCell should look like this:

fn clear_spread(&self, ptr: &mut KeyPtr) {
    let root_key = ExtKeyPtr::next_for::<Self>(ptr);
    match <T as SpreadLayout>::REQUIRES_DEEP_CLEAN_UP {
        true => {
            // Load from storage and then clear:
            clear_spread_root_opt::<T, _>(root_key, || self.get())
        }
        false => {
            // Clear without loading from storage:
            let footprint = <T as SpreadLayout>::FOOTPRINT;
            assert!(footprint <= 16); // magic number
            let mut key_ptr = KeyPtr::from(*root_key);
            for _ in 0..footprint {
                ink_env::clear_contract_storage(key_ptr.advance_by(1));
            }
        }
    }
}