Add cpp_double_fp and exercise arithmetic_tests

[ckormanyos]

The purpose of this pull request is to add the cpp_double_fp_backend<> template class.

In the initial commit, we:

• Run the arithmetic tests.
• Modify elementary function tests to prepare for TEST_CPP_DOUBLE_FLOAT, but do not run them yet.
• Try some further small adaptions to arithmetic_tests.hpp that improve the testing logic for mixed-int128/float128 interaction.

If this runs through, I can add the function tests (stuff like sin(), exp(), etc.) as a next step. These run well and properly in the GSoC fork, but are not yet exercised in this initial commit.

[codecov]

Codecov Report

Attention: Patch coverage is 99.67897% with 4 lines in your changes missing coverage. Please review.

Project coverage is 94.4%. Comparing base (96eb274) to head (ee4da5f).

Files with missing lines	Patch %	Lines
include/boost/multiprecision/cpp_double_fp.hpp	99.6%	3 Missing ⚠️
include/boost/multiprecision/float128.hpp	0.0%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #515     +/-   ##
=========================================
+ Coverage     94.1%   94.4%   +0.3%     
=========================================
  Files          280     293     +13     
  Lines        29074   30287   +1213     
=========================================
+ Hits         27349   28579   +1230     
+ Misses        1725    1708     -17     

Files with missing lines	Coverage Δ
...oost/multiprecision/cpp_df_qf/cpp_df_qf_detail.hpp	100.0% <100.0%> (ø)
...ecision/cpp_df_qf/cpp_df_qf_detail_ccmath_fabs.hpp	100.0% <100.0%> (ø)
...cision/cpp_df_qf/cpp_df_qf_detail_ccmath_floor.hpp	100.0% <100.0%> (ø)
...recision/cpp_df_qf/cpp_df_qf_detail_ccmath_fma.hpp	100.0% <100.0%> (ø)
...cision/cpp_df_qf/cpp_df_qf_detail_ccmath_frexp.hpp	100.0% <100.0%> (ø)
...cision/cpp_df_qf/cpp_df_qf_detail_ccmath_isinf.hpp	100.0% <100.0%> (ø)
...cision/cpp_df_qf/cpp_df_qf_detail_ccmath_isnan.hpp	100.0% <100.0%> (ø)
...cision/cpp_df_qf/cpp_df_qf_detail_ccmath_ldexp.hpp	100.0% <100.0%> (ø)
...recision/cpp_df_qf/cpp_df_qf_detail_ccmath_log.hpp	100.0% <100.0%> (ø)
...ecision/cpp_df_qf/cpp_df_qf_detail_ccmath_sqrt.hpp	100.0% <100.0%> (ø)
... and 27 more

... and 6 files with indirect coverage changes

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