Enhance quality via tests and syntax

[ckormanyos]

The purpose of this PR is to enhance quality via more tests and syntax. Some motivation comes from the new cpp_double_fp_backend backend.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.6%. Comparing base (fcb1f6a) to head (8835179).
Report is 9 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #705     +/-   ##
=========================================
+ Coverage     94.5%   94.6%   +0.1%     
=========================================
  Files          295     295             
  Lines        30492   30687    +195     
=========================================
+ Hits         28788   29000    +212     
+ Misses        1704    1687     -17     

Files with missing lines	Coverage Δ
include/boost/multiprecision/cpp_double_fp.hpp	99.9% <100.0%> (+1.3%)	⬆️
test/test_pow.cpp	98.9% <100.0%> (+0.6%)	⬆️
test/test_various_edges.cpp	100.0% <100.0%> (ø)

... and 3 files with indirect coverage changes

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