Enable 80 and 128 bit long double conversions

[mborland]

I think it's best to not offer __float128 support until demanded based off all the issue that charconv has had because of that decision.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.1%. Comparing base (8274270) to head (833b6e3).
Report is 14 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #756   +/-   ##
=======================================
  Coverage     99.1%   99.1%           
=======================================
  Files          227     227           
  Lines        16863   16863           
  Branches      1816    1816           
=======================================
  Hits         16708   16708           
  Misses         155     155           

Files with missing lines	Coverage Δ
include/boost/decimal/decimal64.hpp	99.3% <100.0%> (ø)
include/boost/decimal/decimal64_fast.hpp	98.6% <ø> (ø)
.../decimal/detail/fast_float/compute_float80_128.hpp	80.0% <100.0%> (ø)
include/boost/decimal/detail/to_float.hpp	100.0% <100.0%> (ø)

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8274270...833b6e3. Read the comment docs.