Added missed WhereClosure for the generics into storage_item

[xgreenx]

Added missed WhereClosure for the generics. Tested this case via documentation

[HCastano]

Do you have a contract which shows what the problem is/was?

If I'd be inclined to add it as a UI test

[codecov-commenter]

Codecov Report

Merging #1536 (f9b34ff) into master (2cd3a0c) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1536      +/-   ##
==========================================
- Coverage   71.62%   71.58%   -0.05%     
==========================================
  Files         205      205              
  Lines        6295     6295              
==========================================
- Hits         4509     4506       -3     
- Misses       1786     1789       +3     

Impacted Files	Coverage Δ
crates/ink/macro/src/lib.rs	100.00% <ø> (ø)
crates/ink/ir/src/ir/storage_item/mod.rs	75.00% <100.00%> (+1.08%)	⬆️
crates/allocator/src/bump.rs	88.23% <0.00%> (-1.85%)	⬇️
crates/ink/ir/src/ir/attrs.rs	81.99% <0.00%> (-0.28%)	⬇️

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[xgreenx]

Previously it was not possible=)

[ascjones]

LGTM, the doc tests are good enough to prove this works