feat: add custom attributes to `fn` items #2893

Job	Run time
docs	17s
minimal	16s
check-cfg	18s
msrv	24s
quickchecking	17s
rustfmt-clippy	12s
test-book	15s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 8s
test-expectations (ubuntu-latest)	41s
test-no-headers	27s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	58s
test-expectations (macos-latest)	2m 31s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	57s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 7s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	55s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	1m 7s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	52s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	1m 1s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	56s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	55s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	55s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	55s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	56s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	54s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	54s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	55s
test (ubuntu-latest, 16.0, 0, 0, 1)	55s
test (macos-latest, 16.0, 0, 0, 0)	55s
success	1s
	22m 54s

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