Reorganize tests and do some cleanup

[idubrov]

Also, remove from_value function as serde_json::from_value can be used instead (breaking change).

[codecov[bot]]

Codecov Report

Merging #23 (fb3529b) into main (472c41c) will decrease coverage by 0.53%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
- Coverage   96.62%   96.08%   -0.54%     
==========================================
  Files           2        2              
  Lines         178      179       +1     
==========================================
  Hits          172      172              
- Misses          6        7       +1     

Impacted Files	Coverage Δ
src/diff.rs	93.87% <ø> (+2.04%)	:arrow_up:
src/lib.rs	96.92% <0.00%> (-1.53%)	:arrow_down:

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