Add simplified key hasher

[mberr]

This PR adds a simplification for creating hashers based on the values associated to a subse of keys without having to define a lambda or named function.

[codecov-commenter]

Codecov Report

Merging #3 (a86487b) into main (7604483) will increase coverage by 2.57%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
+ Coverage   47.42%   50.00%   +2.57%     
==========================================
  Files           3        3              
  Lines          97      104       +7     
  Branches       16       19       +3     
==========================================
+ Hits           46       52       +6     
  Misses         42       42              
- Partials        9       10       +1     

Impacted Files	Coverage Δ
src/torch_max_mem/version.py	0.00% <ø> (ø)
src/torch_max_mem/api.py	56.32% <88.88%> (+2.57%)	⬆️

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