Fix for issue #166 negative precision

[mborland]

Partially Addresses: #166

Adds additional overloads to floating point to_chars to change the behavior based on unspecified vs negative precision

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (f5e1971) 92.30% compared to head (340a72e) 92.32%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #168      +/-   ##
===========================================
+ Coverage    92.30%   92.32%   +0.02%     
===========================================
  Files           64       65       +1     
  Lines         8316     8340      +24     
===========================================
+ Hits          7676     7700      +24     
  Misses         640      640              

Files	Coverage Δ
include/boost/charconv/to_chars.hpp	100.00% <ø> (ø)
src/to_chars.cpp	93.44% <100.00%> (+0.85%)	⬆️
test/github_issue_166.cpp	100.00% <100.00%> (ø)
src/to_chars_float_impl.hpp	80.86% <89.65%> (+1.30%)	⬆️

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