Change `Array.Get()` to return `Value` instead of `Storable`

[fxamacker]

Closes #315 Updates #296 #292

Description

This PR changes Array.Get() to return Value instead of Storable.

Currently, Array.Get() returns Storable and client converts returned Storable to Value. However, it only makes sense to return Storable if client needs to remove register (slab) by StorageID (StorageIDStorable).

Get() should only provide Value without possibility of client manipulating the underlying Storable.

This is prep work for Atree Register Inlining (#292) and will also harden the API.

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• [x] Targeted PR against main branch
• [x] Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• [x] Code follows the standards mentioned here
• [x] Updated relevant documentation
• [x] Re-reviewed Files changed in the Github PR explorer
• [x] Added appropriate labels

[codecov-commenter]

Codecov Report

Merging #316 (044db01) into main (7c97278) will decrease coverage by 0.03%. The diff coverage is 66.66%.

@@            Coverage Diff             @@
##             main     #316      +/-   ##
==========================================
- Coverage   64.62%   64.60%   -0.03%     
==========================================
  Files          14       14              
  Lines        7991     8000       +9     
==========================================
+ Hits         5164     5168       +4     
- Misses       2152     2155       +3     
- Partials      675      677       +2     

Impacted Files	Coverage Δ
array.go	69.80% <66.66%> (-0.07%)	:arrow_down:

... and 2 files with indirect coverage changes