Change digest level from int to uint

[fxamacker]

Description

Digest level must be equal to larger than 0. By changing its data type from int to uint, we can avoid the possiblity of negative value.

Closes #256

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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 #257 (ec8d0e4) into main (5494f9e) will decrease coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #257      +/-   ##
==========================================
- Coverage   68.97%   68.94%   -0.03%     
==========================================
  Files          14       14              
  Lines        7245     7245              
==========================================
- Hits         4997     4995       -2     
- Misses       1617     1619       +2     
  Partials      631      631              

Impacted Files	Coverage Δ
map_debug.go	55.50% <ø> (ø)
hash.go	71.83% <100.00%> (ø)
map.go	71.41% <100.00%> (ø)
storage.go	79.28% <0.00%> (-0.36%)	:arrow_down:

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[fxamacker]

As changing signed to unsigned makes underflow a little "more common" (e.g. decrement of 0 now underflows/wraps around), I checked all changes, and as far as I can see, the code only ever seems to increment the levels. Maybe good to double check though, e.g. that we do not have something like a comparison on something like level-1, like in the other PR

Good point! I just double checked and it looks good.