refactor: `expr_map_impl` now caches the value #26

Job	Run time
std::format none C++20 Clang-18 Release	6m 23s
std::format none C++20 Clang-18 Debug	6m 5s
std::format none C++23 Clang-18 Release	5m 18s
std::format none C++23 Clang-18 Debug	6m 4s
std::format gsl-lite C++20 Clang-18 Release	6m 35s
std::format gsl-lite C++20 Clang-18 Debug	6m 11s
std::format gsl-lite C++23 Clang-18 Release	6m 30s
std::format gsl-lite C++23 Clang-18 Debug	6m 26s
std::format ms-gsl C++20 Clang-18 Release	6m 30s
std::format ms-gsl C++20 Clang-18 Debug	5m 3s
std::format ms-gsl C++23 Clang-18 Release	5m 32s
std::format ms-gsl C++23 Clang-18 Debug	6m 3s
fmtlib none C++20 Clang-18 Release	6m 31s
fmtlib none C++20 Clang-18 Debug	6m 15s
fmtlib none C++23 Clang-18 Release	6m 28s
fmtlib none C++23 Clang-18 Debug	6m 2s
fmtlib gsl-lite C++20 Clang-18 Release	6m 49s
fmtlib gsl-lite C++20 Clang-18 Debug	6m 15s
fmtlib gsl-lite C++23 Clang-18 Release	6m 31s
fmtlib gsl-lite C++23 Clang-18 Debug	6m 8s
fmtlib ms-gsl C++20 Clang-18 Release	6m 44s
fmtlib ms-gsl C++20 Clang-18 Debug	6m 18s
fmtlib ms-gsl C++23 Clang-18 Release	6m 28s
fmtlib ms-gsl C++23 Clang-18 Debug	6m 28s
	2h 29m 37s

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