Fix cohort handling of zero to match IEEE 754 requirements

[mborland]

Closes: #1387

[mborland]

These 32bit MSVC 14.5 failures are bizarre. It loses the sign and has stack overflows all over the place where the other MSVC versions have been green for a while. I'm going to move adding that runner to a different PR so that I can investigate what's going on better. These changes are still the correct ones.

[codecov]

Codecov Report

❌ Patch coverage is 87.61905% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.5%. Comparing base (2b920bf) to head (54c3fb1).
⚠️ Report is 15 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/decimal128_t.hpp	66.7%	2 Missing ⚠️
include/boost/decimal/decimal32_t.hpp	66.7%	2 Missing ⚠️
include/boost/decimal/decimal_fast128_t.hpp	81.9%	2 Missing ⚠️
include/boost/decimal/decimal_fast32_t.hpp	81.9%	2 Missing ⚠️
...ude/boost/decimal/detail/remove_trailing_zeros.hpp	66.7%	2 Missing ⚠️
include/boost/decimal/decimal64_t.hpp	83.4%	1 Missing ⚠️
include/boost/decimal/decimal_fast64_t.hpp	91.0%	1 Missing ⚠️
include/boost/decimal/detail/parser.hpp	80.0%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1389     +/-   ##
=========================================
- Coverage     98.5%   98.5%   -0.0%     
=========================================
  Files          288     288             
  Lines        22749   22831     +82     
  Branches      1971    1985     +14     
=========================================
+ Hits         22402   22472     +70     
- Misses         347     359     +12     

Files with missing lines	Coverage Δ
include/boost/decimal/charconv.hpp	95.9% <100.0%> (ø)
include/boost/decimal/detail/add_impl.hpp	94.0% <100.0%> (+0.7%)	⬆️
include/boost/decimal/detail/comparison.hpp	96.8% <100.0%> (+0.1%)	⬆️
include/boost/decimal/detail/div_impl.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/quantize_impl.hpp	94.9% <ø> (+4.7%)	⬆️
test/github_issue_1383.cpp	100.0% <100.0%> (ø)
test/test_cmath.cpp	100.0% <100.0%> (ø)
test/test_erf.cpp	100.0% <100.0%> (ø)
test/test_to_chars.cpp	100.0% <100.0%> (ø)
include/boost/decimal/decimal64_t.hpp	94.6% <83.4%> (-0.1%)	⬇️
... and 7 more

... and 1 file with indirect coverage changes

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