Add SpreadAllocate and PackedAllocate traits and impls

[Robbepop]

Implements #961.

[codecov-commenter]

Codecov Report

Merging #978 (093e580) into master (4534c4a) will decrease coverage by 16.42%.
The diff coverage is 47.82%.

@@             Coverage Diff             @@
##           master     #978       +/-   ##
===========================================
- Coverage   79.16%   62.74%   -16.43%     
===========================================
  Files         244      244               
  Lines        9149     9160       +11     
===========================================
- Hits         7243     5747     -1496     
- Misses       1906     3413     +1507     

Impacted Files	Coverage Δ
crates/storage/src/collections/stash/mod.rs	97.43% <ø> (ø)
crates/storage/src/collections/stash/storage.rs	60.97% <0.00%> (-4.82%)	⬇️
crates/storage/src/collections/vec/storage.rs	64.70% <ø> (ø)
crates/storage/src/lazy/cache_cell.rs	100.00% <ø> (ø)
crates/storage/src/lazy/entry.rs	94.73% <ø> (ø)
crates/storage/src/lazy/lazy_array.rs	96.22% <ø> (ø)
crates/storage/src/lazy/lazy_hmap.rs	93.23% <ø> (ø)
crates/storage/src/lazy/lazy_imap.rs	92.94% <ø> (ø)
crates/storage/src/lazy/mod.rs	93.54% <ø> (ø)
crates/storage/src/memory.rs	100.00% <ø> (ø)
... and 69 more

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[cmichi]

Thanks, lgtm!

We should definitely add tests in a follow-up though! Could you create a follow-up issue for that?