Improve coverage and code

[h4sh3d]

Improve coverage and code for encode_block and decode_block. Fix a bug in the condition that validates inputs.

[codecov]

Codecov Report

Merging #13 (629642b) into main (dc44b61) will increase coverage by 0.69%.
The diff coverage is 100.00%.

❗ Current head 629642b differs from pull request most recent head a38b7f6. Consider uploading reports for the commit a38b7f6 to get more accurate results

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
+ Coverage   94.57%   95.26%   +0.69%     
==========================================
  Files           1        1              
  Lines         516      528      +12     
==========================================
+ Hits          488      503      +15     
+ Misses         28       25       -3     

Impacted Files	Coverage Δ
src/base58.rs	95.26% <100.00%> (+0.69%)	⬆️

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