Allow mixed keys

[simPod]

My cache allows keys of any type. Can we narrow the type of key?

[codecov-commenter]

Codecov Report

Merging #59 (ac2e757) into master (4347804) will increase coverage by 0.91%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
+ Coverage   56.61%   57.53%   +0.91%     
==========================================
  Files           7        7              
  Lines         325      332       +7     
==========================================
+ Hits          184      191       +7     
  Misses        141      141              

Impacted Files	Coverage Δ
src/Option.php	100.00% <0.00%> (ø)
...romise-adapter/src/Adapter/ReactPromiseAdapter.php	0.00% <0.00%> (ø)
...e-adapter/src/Adapter/GuzzleHttpPromiseAdapter.php	0.00% <0.00%> (ø)
...r/src/Adapter/WebonyxGraphQLSyncPromiseAdapter.php	0.00% <0.00%> (ø)
src/DataLoader.php	99.33% <0.00%> (+0.02%)	⬆️

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[mcg-web]

Look good to me. Thank you @simPod !