Fix various warnings

[mborland]

Closes: #1291

[codecov]

Codecov Report

❌ Patch coverage is 97.10145% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.9%. Comparing base (9489bbd) to head (5d73132).
⚠️ Report is 9 commits behind head on develop.

Files with missing lines	Patch %	Lines
include/boost/decimal/decimal128_t.hpp	0.0%	1 Missing ⚠️
include/boost/decimal/decimal_fast128_t.hpp	0.0%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #1293     +/-   ##
=========================================
- Coverage     99.0%   98.9%   -0.0%     
=========================================
  Files          268     268             
  Lines        17754   17823     +69     
  Branches      1984    1985      +1     
=========================================
+ Hits         17561   17624     +63     
- Misses         193     199      +6     

Files with missing lines	Coverage Δ
include/boost/decimal/cstdio.hpp	97.6% <ø> (ø)
include/boost/decimal/decimal32_t.hpp	96.7% <100.0%> (ø)
include/boost/decimal/decimal64_t.hpp	95.3% <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/cmath/atan.hpp	100.0% <100.0%> (+3.6%)	⬆️
include/boost/decimal/detail/cmath/cos.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/cmath/exp.hpp	100.0% <100.0%> (ø)
include/boost/decimal/detail/cmath/frexp.hpp	92.0% <100.0%> (-0.3%)	⬇️
include/boost/decimal/detail/cmath/ilogb.hpp	100.0% <ø> (ø)
... and 80 more

... and 22 files with indirect coverage changes

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