support int8_t; update README

[bbbgan]

No description provided.

[qicosmos]

我担心这个处理会不会太特殊了？会破坏最初的语义？是否应该加一个宏，只有开启才这样处理会不会好点？

[bbbgan]

没有吧，不会破坏什么，只是之前一直没注意

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.10% 🎉

Comparison is base (37c363e) 43.91% compared to head (b1b8b82) 44.01%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #210      +/-   ##
==========================================
+ Coverage   43.91%   44.01%   +0.10%     
==========================================
  Files          45       45              
  Lines        5363     5373      +10     
==========================================
+ Hits         2355     2365      +10     
  Misses       3008     3008              

Files Changed	Coverage Δ
iguana/detail/charconv.h	87.50% <100.00%> (+4.16%)	⬆️
test/unit_test.cpp	99.65% <100.00%> (+<0.01%)	⬆️

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[qicosmos]

LGTM