Add fmt::is_contiguous<std::basic_string<Char, Traits, Allocator>> #5509

Job	Run time
build (g++-4.8, Debug, 11)	1m 17s
build (g++-4.8, Release, 11)	1m 45s
build (g++-10, Debug, 11)	1m 10s
build (g++-10, Release, 11)	1m 55s
build (clang++-9, Debug, 11)	1m 16s
build (clang++-9, Release, 11)	2m 1s
build (g++-8, Debug, 14, sudo apt install g++-8)	1m 16s
build (g++-8, Debug, 17, sudo apt install g++-8)	1m 25s
build (g++-9, Debug, 17)	1m 37s
build (g++-10, Debug, 17)	1m 26s
build (g++-11, Debug, 20, sudo apt install g++-11)	1m 48s
build (clang++-8, Debug, 17, -stdlib=libc++, sudo apt install clang-8 libc++-8-dev libc++abi-8-dev)	1m 26s
build (clang++-9, Debug, 17, -DFMT_FUZZ=ON -DFMT_FUZZ_LINKMAIN=ON, sudo apt install clang-9)	1m 40s
build (clang++-11, Debug, 20)	1m 50s
build (clang++-11, Debug, 20, -stdlib=libc++, sudo apt install libc++-11-dev libc++abi-11-dev)	1m 37s
	23m 29s

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