Make sure void is idempotent

[johnynek]

This adds a law (that actually already passed), and makes unmap a bit more precise in the case of Defer.

[codecov-commenter]

Codecov Report

Merging #297 (857126a) into main (eea17fa) will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #297      +/-   ##
==========================================
+ Coverage   96.24%   96.53%   +0.29%     
==========================================
  Files           8        8              
  Lines        1011     1011              
  Branches       81       81              
==========================================
+ Hits          973      976       +3     
+ Misses         38       35       -3     

Impacted Files	Coverage Δ
core/shared/src/main/scala/cats/parse/Parser.scala	96.44% <100.00%> (+0.35%)	⬆️

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[regadas]

Makes sense!