Make -0 = +0

[mborland]

This is a property guaranteed by IEEE 754

Closes: #811

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (30ed86c) to head (f25014b).
Report is 13 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #815     +/-   ##
=========================================
- Coverage     98.9%   98.9%   -0.0%     
=========================================
  Files          228     228             
  Lines        16978   16995     +17     
  Branches      1810    1809      -1     
=========================================
+ Hits         16787   16803     +16     
- Misses         191     192      +1     

Files with missing lines	Coverage Δ
include/boost/decimal/decimal128.hpp	97.8% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal128_fast.hpp	98.3% <100.0%> (-<0.1%)	⬇️
include/boost/decimal/decimal32_fast.hpp	96.5% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal64_fast.hpp	98.2% <100.0%> (ø)
include/boost/decimal/detail/comparison.hpp	99.4% <100.0%> (+0.1%)	⬆️
test/random_decimal128_comp.cpp	100.0% <100.0%> (ø)
test/random_decimal128_fast_comp.cpp	100.0% <100.0%> (ø)
test/random_decimal32_comp.cpp	100.0% <100.0%> (ø)
test/random_decimal32_fast_comp.cpp	100.0% <100.0%> (ø)
test/random_decimal64_comp.cpp	100.0% <100.0%> (ø)
... and 1 more

... and 2 files with indirect coverage changes

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