use `<iosfwd>` in header #443

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Run time

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Job	Run time
build (clang++, ubuntu-20.04)	16s
build (clang++, ubuntu-22.04)	1m 59s
build (clang++, macos-11)	1s
build (clang++, macos-12)	38s
build (g++, ubuntu-20.04)	20s
build (g++, ubuntu-22.04)	2m 2s
build (g++, macos-11)	1s
build (g++, macos-12)	45s
	6m 2s