Make Vector usable when initialized with SpreadAllocate

[HCastano]

At the moment the ink_storage::Vec type is unusable when initialized using
SpreadAllocate::allocate_spread. This is because Vec::len does not account that the
vector can be in a state where the it is initialized but there is nothing in storage.

This creates problems when using Vec::len, such as during Drop (see here),
or Vec::push.

This PR changes the behaviour of Vec::len() to return a 0 length if there is no
length in storage. It also changes all uses of Lazy::get_mut for modifying Vec.len to
use Lazy::set instead since that doesn't panic if no item exists in storage.

Currently a draft since there are some other storage data structures which use Vec
whose tests need to be updated and I'm not quite sure what the best way to fix those
tests is just yet.

[cmichi]

@HCastano I can't think of a better approach, so I'm fine with the PR if you fix the other tests.

@ascjones Do you have an opinion?

[codecov-commenter]

Codecov Report

Merging #1070 (6e3b7e3) into master (9b154e8) will decrease coverage by 15.93%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master    #1070       +/-   ##
===========================================
- Coverage   78.75%   62.82%   -15.94%     
===========================================
  Files         248      248               
  Lines        9371     9371               
===========================================
- Hits         7380     5887     -1493     
- Misses       1991     3484     +1493     

Impacted Files	Coverage Δ
crates/storage/src/alloc/tests.rs	100.00% <ø> (ø)
...rates/storage/src/collections/binary_heap/tests.rs	96.96% <ø> (ø)
crates/storage/src/collections/bitstash/tests.rs	100.00% <ø> (ø)
crates/storage/src/collections/bitvec/tests.rs	100.00% <ø> (ø)
crates/storage/src/collections/vec/tests.rs	100.00% <ø> (ø)
crates/storage/src/collections/vec/mod.rs	97.29% <100.00%> (ø)
crates/storage/src/lazy/mod.rs	94.44% <100.00%> (+9.15%)	⬆️
crates/lang/codegen/src/traits.rs	0.00% <0.00%> (-100.00%)	⬇️
crates/lang/ir/src/ir/contract.rs	0.00% <0.00%> (-100.00%)	⬇️
crates/lang/codegen/src/generator/env.rs	0.00% <0.00%> (-100.00%)	⬇️
... and 61 more

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[HCastano]

We got rid of ink_storage::Vec in #1111, so I'm gonna close this