Enable string constructor to parse payloads of NANs

[mborland]

Closes: #1184

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.2%. Comparing base (c6dd35a) to head (14d4bbf).
⚠️ Report is 37 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1219     +/-   ##
=========================================
+ Coverage     99.2%   99.2%   +0.1%     
=========================================
  Files          257     259      +2     
  Lines        17134   17193     +59     
  Branches      1935    1941      +6     
=========================================
+ Hits         16994   17054     +60     
+ Misses         140     139      -1     

Files with missing lines	Coverage Δ
include/boost/decimal/charconv.hpp	97.6% <100.0%> (+0.1%)	⬆️
include/boost/decimal/decimal_fast128_t.hpp	96.0% <ø> (ø)
include/boost/decimal/decimal_fast32_t.hpp	100.0% <ø> (ø)
include/boost/decimal/decimal_fast64_t.hpp	100.0% <ø> (ø)
include/boost/decimal/detail/cmath/nan.hpp	92.4% <100.0%> (-0.5%)	⬇️
include/boost/decimal/detail/parser.hpp	96.3% <100.0%> (+0.4%)	⬆️
include/boost/decimal/detail/write_payload.hpp	100.0% <100.0%> (ø)
test/github_issue_1054.cpp	100.0% <100.0%> (ø)
test/test_from_chars_nan_payloads.cpp	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes

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