Make float128 a trivially copyable type

[nmnobre]

Closes #635.

Opted to go for trivially copyable and not fully trivial as otherwise we'd have to make the constexpr float128_backend() noexcept constructor trivial but I expect we'd like to keep zero initialisation for the data member m_value we're wrapping even when using default initialisation. In any case, I never needed the type to be trivial, only trivially copyable, so I'm happy if you are?

Code snippet to print type traits and limits: https://godbolt.org/z/ddsj16M7W. I'm not sure we need everything in there, I just copied the original snippet and added a trivially copyable check, but feel free to edit it to your liking and we can then amend the website. :)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.1%. Comparing base (0f6a276) to head (320d8b7).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop    #636     +/-   ##
=========================================
- Coverage     94.1%   94.1%   -0.0%     
=========================================
  Files          276     276             
  Lines        26846   26839      -7     
=========================================
- Hits         25240   25232      -8     
- Misses        1606    1607      +1     

Files	Coverage Δ
include/boost/multiprecision/number.hpp	96.5% <100.0%> (-<0.1%)	⬇️

... and 1 file with indirect coverage changes

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[mborland]

Thanks for this. I also added a trivial test: c2b11c1, and update the code snippet from your original question: b16cff2