Optimize string, void and char cases a bit

[johnynek]

related to #64 here are some special cases to make sure we don't harm efficiency when converting between original, string, and void return types.

cc @rossabaker

[codecov-io]

Codecov Report

Merging #67 (72acc37) into main (5000dbe) will decrease coverage by 0.27%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
- Coverage   97.11%   96.83%   -0.28%     
==========================================
  Files           3        3              
  Lines         589      601      +12     
  Branches       52       55       +3     
==========================================
+ Hits          572      582      +10     
- Misses         17       19       +2     

Impacted Files	Coverage Δ
core/shared/src/main/scala/cats/parse/Parser.scala	97.65% <96.29%> (-0.32%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5000dbe...72acc37. Read the comment docs.