minor improvement to cheapOneOf0

[johnynek]

We were potentially hiding a OneOf by not carefully checking if each argument might be a Parser.

[codecov-commenter]

Codecov Report

Merging #400 (1b7c7fe) into main (fe8adc5) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #400      +/-   ##
==========================================
+ Coverage   96.72%   96.73%   +0.01%     
==========================================
  Files           9        9              
  Lines        1284     1288       +4     
  Branches      124      133       +9     
==========================================
+ Hits         1242     1246       +4     
  Misses         42       42              

Impacted Files	Coverage Δ
core/shared/src/main/scala/cats/parse/Parser.scala	96.44% <100.00%> (+0.01%)	⬆️

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