feat: `iec::bit` using-declared in `iec::unit_symbols` #221

Job	Run time
std::format none C++20 Clang-18 Release	3m 23s
std::format none C++20 Clang-18 Debug	4m 48s
std::format none C++23 Clang-18 Release	4m 26s
std::format none C++23 Clang-18 Debug	4m 31s
std::format gsl-lite C++20 Clang-18 Release	4m 59s
std::format gsl-lite C++20 Clang-18 Debug	5m 9s
std::format gsl-lite C++23 Clang-18 Release	4m 54s
std::format gsl-lite C++23 Clang-18 Debug	4m 40s
std::format ms-gsl C++20 Clang-18 Release	4m 56s
std::format ms-gsl C++20 Clang-18 Debug	4m 48s
std::format ms-gsl C++23 Clang-18 Release	4m 28s
std::format ms-gsl C++23 Clang-18 Debug	4m 40s
fmtlib none C++20 Clang-18 Release	7m 25s
fmtlib none C++20 Clang-18 Debug	7m 1s
fmtlib none C++23 Clang-18 Release	7m 5s
fmtlib none C++23 Clang-18 Debug	6m 34s
fmtlib gsl-lite C++20 Clang-18 Release	7m 28s
fmtlib gsl-lite C++20 Clang-18 Debug	7m 2s
fmtlib gsl-lite C++23 Clang-18 Release	7m 11s
fmtlib gsl-lite C++23 Clang-18 Debug	6m 49s
fmtlib ms-gsl C++20 Clang-18 Release	7m 29s
fmtlib ms-gsl C++20 Clang-18 Debug	7m 9s
fmtlib ms-gsl C++23 Clang-18 Release	8m 0s
fmtlib ms-gsl C++23 Clang-18 Debug	6m 54s
	2h 21m 49s

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