Use native C++20 feature for bit_cast if available

[axic]

Fixes https://github.com/wasmx/fizzy/pull/535/files#r492656891.

[chfast]

This is not doable right now because only MSVC has implemented <bit> so far. See "std::bit_cast" in https://en.cppreference.com/w/cpp/compiler_support#C.2B.2B20_library_features.

[axic]

This is not doable right now because only MSVC has implemented so far. See "std::bit_cast" in https://en.cppreference.com/w/cpp/compiler_support#C.2B.2B20_library_features.

Well we could use this other macro:

2.5. Feature testing
The __cpp_lib_bit_cast feature test macro should be added.

But since we don't support compilation with MSVC it can't be tested.

[chfast]

This is not doable right now because only MSVC has implemented <bit> so far. See "std::bit_cast" in https://en.cppreference.com/w/cpp/compiler_support#C.2B.2B20_library_features.

This still holds.

[codecov]

Codecov Report

Merging #542 (d8f3fe8) into master (7901fe9) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head d8f3fe8 differs from pull request most recent head ba205ff. Consider uploading reports for the commit ba205ff to get more accurate results

@@            Coverage Diff             @@
##           master     #542      +/-   ##
==========================================
- Coverage   99.12%   99.10%   -0.02%     
==========================================
  Files          81       81              
  Lines       12985    12941      -44     
==========================================
- Hits        12871    12825      -46     
- Misses        114      116       +2     

Flag	Coverage Δ
rust	99.90% <ø> (ø)
spectests	?
unittests	?
unittests-32	99.03% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
lib/fizzy/cxx20/bit.hpp	100.00% <ø> (ø)
test/testfloat/testfloat.cpp	95.86% <0.00%> (-1.38%)	⬇️
test/unittests/execute_floating_point_test.cpp	99.70% <0.00%> (-0.02%)	⬇️
lib/fizzy/execute.cpp	99.29% <0.00%> (-0.01%)	⬇️
test/unittests/instantiate_test.cpp	100.00% <0.00%> (ø)
test/unittests/floating_point_utils_test.cpp	100.00% <0.00%> (ø)

[axic]

This is not doable right now because only MSVC has implemented so far. See "std::bit_cast" in https://en.cppreference.com/w/cpp/compiler_support#C.2B.2B20_library_features.

This still holds.

Totally skipped over this fact.

[axic]

This is waiting for GCC+11 now: https://www.gnu.org/software/gcc/gcc-11/changes.html

[axic]

GCC 11 and Clang 14 has this now. Since #804 at least gcc is of correct version.

[axic]

Tested with introduced a compilation error that the code is actually compiled on gcc 11.

[axic]

@chfast @gumb0 please review