Re-work architecture of charconv to fix sting construction with old clangs

[mborland]

Closes: #1229

[codecov]

Codecov Report

❌ Patch coverage is 98.83721% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 99.1%. Comparing base (00d6406) to head (dc5b705).
⚠️ Report is 22 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/detail/from_chars_impl.hpp	96.5%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #1231   +/-   ##
=======================================
  Coverage     99.1%   99.1%           
=======================================
  Files          263     264    +1     
  Lines        17251   17251           
  Branches      1940    1931    -9     
=======================================
  Hits         17079   17079           
  Misses         172     172           

Files with missing lines	Coverage Δ
include/boost/decimal/charconv.hpp	97.5% <ø> (+0.1%)	⬆️
include/boost/decimal/decimal128_t.hpp	98.2% <100.0%> (ø)
include/boost/decimal/decimal32_t.hpp	96.6% <100.0%> (ø)
include/boost/decimal/decimal64_t.hpp	95.2% <100.0%> (ø)
include/boost/decimal/decimal_fast128_t.hpp	96.0% <100.0%> (ø)
include/boost/decimal/decimal_fast32_t.hpp	100.0% <100.0%> (ø)
include/boost/decimal/decimal_fast64_t.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/locale_conversion.hpp	98.3% <ø> (ø)
include/boost/decimal/detail/from_chars_impl.hpp	96.5% <96.5%> (ø)

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