Sign change warning fix

[henryiii]

Fix #367.

Also using std::size_t instead of size_t since this is C++ not C.

[codecov]

Codecov Report

Merging #382 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #382   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          12     12           
  Lines        3366   3366           
=====================================
  Hits         3366   3366

Impacted Files	Coverage Δ
include/CLI/Split.hpp	100% <ø> (ø)	⬆️
include/CLI/Timer.hpp	100% <100%> (ø)	⬆️
include/CLI/Validators.hpp	100% <100%> (ø)	⬆️
include/CLI/TypeTools.hpp	100% <100%> (ø)	⬆️
include/CLI/Option.hpp	100% <100%> (ø)	⬆️
include/CLI/App.hpp	100% <100%> (ø)	⬆️
include/CLI/StringTools.hpp	100% <100%> (ø)	⬆️
include/CLI/FormatterFwd.hpp	100% <100%> (ø)	⬆️
include/CLI/ConfigFwd.hpp	100% <100%> (ø)	⬆️
include/CLI/Error.hpp	100% <100%> (ø)	⬆️

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