Remove sprintf and fix conceptual requirements for <cstdio>

[mborland]

Closes: #803

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.9%. Comparing base (30ed86c) to head (ad00b1e).
Report is 7 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop    #813   +/-   ##
=======================================
  Coverage     98.9%   98.9%           
=======================================
  Files          228     228           
  Lines        16978   16978           
  Branches      1810    1809    -1     
=======================================
  Hits         16787   16787           
  Misses         191     191           

Files with missing lines	Coverage Δ
include/boost/decimal/cstdio.hpp	95.6% <ø> (ø)

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