[storage] Implement Drop for Pack

[cmichi]

Closes #581.

[codecov-io]

Codecov Report

Merging #600 (ad010af) into master (2100fa7) will increase coverage by 15.18%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master     #600       +/-   ##
===========================================
+ Coverage   67.26%   82.44%   +15.18%     
===========================================
  Files         155      155               
  Lines        6925     6939       +14     
===========================================
+ Hits         4658     5721     +1063     
+ Misses       2267     1218     -1049     

Impacted Files	Coverage Δ
...rates/storage/src/collections/binary_heap/tests.rs	94.00% <ø> (ø)
crates/storage/src/collections/hashmap/tests.rs	100.00% <ø> (ø)
crates/storage/src/collections/stash/tests.rs	100.00% <ø> (ø)
crates/primitives/src/key_ptr.rs	100.00% <100.00%> (ø)
crates/storage/src/pack.rs	100.00% <100.00%> (ø)
crates/lang/ir/src/ir/attrs.rs	85.58% <0.00%> (+0.93%)	⬆️
crates/storage/src/collections/stash/storage.rs	59.45% <0.00%> (+5.40%)	⬆️
...ates/storage/src/collections/hashmap/fuzz_tests.rs	100.00% <0.00%> (+5.47%)	⬆️
crates/env/src/engine/off_chain/db/events.rs	16.12% <0.00%> (+6.45%)	⬆️
crates/lang/ir/src/ir/trait_def.rs	92.82% <0.00%> (+9.74%)	⬆️
... and 28 more

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[Robbepop]

All in all nice fix and the impl mostly looks good, however, I really wonder about the BinaryHeap change.