support yaml 17

[bbbgan]

YAML is supported in C++17; C++20 performance remains nearly unchanged. However, C++20 serialization outperforms C++17.

from left to right, it is: current repository, C++20 of PR , C++17 of PR.

[codecov-commenter]

Codecov Report

Patch coverage: 62.50% and project coverage change: +0.20% 🎉

Comparison is base (4f81eeb) 43.70% compared to head (896cf64) 43.91%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #209      +/-   ##
==========================================
+ Coverage   43.70%   43.91%   +0.20%     
==========================================
  Files          45       45              
  Lines        5343     5363      +20     
==========================================
+ Hits         2335     2355      +20     
  Misses       3008     3008              

Files Changed	Coverage Δ
example/yaml_example.cpp	0.00% <ø> (ø)
iguana/yaml_util.hpp	0.00% <0.00%> (ø)
iguana/yaml_writer.hpp	100.00% <ø> (ø)
iguana/yaml_reader.hpp	100.00% <100.00%> (ø)
test/test_yaml.cpp	100.00% <100.00%> (ø)

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[qicosmos]

LGTM