Add buffer sizing for charconv

[mborland]

Closes: #509

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.8%. Comparing base (3ebfb24) to head (38f6249).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #562     +/-   ##
=========================================
+ Coverage     98.8%   98.8%   +0.1%     
=========================================
  Files          192     192             
  Lines        11480   11486      +6     
  Branches      1389    1389             
=========================================
+ Hits         11341   11347      +6     
  Misses         139     139             

Files	Coverage Δ
include/boost/decimal/charconv.hpp	94.9% <ø> (ø)
test/test_to_chars.cpp	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 3ebfb24...38f6249. Read the comment docs.