Add hexdigest URL processor

[cthoyt]

Requested by @kkaris

[codecov-commenter]

Codecov Report

Merging #35 (23df3af) into main (0754281) will increase coverage by 0.41%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #35      +/-   ##
==========================================
+ Coverage   64.01%   64.42%   +0.41%     
==========================================
  Files          10       10              
  Lines         778      787       +9     
  Branches      131      133       +2     
==========================================
+ Hits          498      507       +9     
  Misses        260      260              
  Partials       20       20              

Impacted Files	Coverage Δ
src/pystow/utils.py	60.85% <100.00%> (+1.29%)	⬆️

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