Do not assume char is always signed

[erik-hallberg]

This is platform-dependent and having it wrong makes the PRId8 and PRIi8 macros print int8_t values as unsigned on some platforms.

[codecov-io]

Codecov Report

Merging #61 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #61   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines         359    359           
=====================================
  Hits          359    359

Impacted Files	Coverage Δ
printf.c	100% <100%> (ø)	⬆️

---

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[dublevskyi]

Can you please merge the PR? @mpaland

[eyalroz]

@dublevskyi : I've merged this into my fork, which is basically a merge of a bunch of small changes and fixes people have suggested.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@            Coverage Diff             @@
##             master       #61   +/-   ##
==========================================
  Coverage          ?   100.00%           
==========================================
  Files             ?         1           
  Lines             ?       359           
  Branches          ?         0           
==========================================
  Hits              ?       359           
  Misses            ?         0           
  Partials          ?         0           

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