[fix] add more missing exports

[biojppm]

re biojppm/rapidyaml#182

[codecov]

Codecov Report

Merging #57 (ba9f86d) into master (7937c59) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #57   +/-   ##
=======================================
  Coverage   92.51%   92.51%           
=======================================
  Files          50       50           
  Lines        9825     9825           
=======================================
  Hits         9090     9090           
  Misses        735      735           

Impacted Files	Coverage Δ
src/c4/error.hpp	100.00% <ø> (ø)
src/c4/format.hpp	96.26% <ø> (ø)

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