[impl] add decoding of utf codepoints

[biojppm]

re biojppm/rapidyaml#204

[codecov]

Codecov Report

Merging #65 (e639878) into master (0a5d475) will increase coverage by 0.03%.
The diff coverage is 97.77%.

@@            Coverage Diff             @@
##           master      #65      +/-   ##
==========================================
+ Coverage   93.16%   93.19%   +0.03%     
==========================================
  Files          49       51       +2     
  Lines       10089    10134      +45     
==========================================
+ Hits         9399     9444      +45     
  Misses        690      690              

Impacted Files	Coverage Δ
src/c4/charconv.hpp	97.58% <ø> (ø)
src/c4/utf.cpp	96.96% <96.96%> (ø)
test/test_utf.cpp	100.00% <100.00%> (ø)
src/c4/substr.hpp	98.98% <0.00%> (+0.14%)	⬆️

---

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 0a5d475...e639878. Read the comment docs.