feat: add custom attributes to `fn` items #2902

Job	Run time
docs	3s
minimal	7s
check-cfg	11s
msrv	10s
quickchecking	8s
rustfmt-clippy	14s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	33s
test-book	11s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	39s
test-no-headers	4s
test-expectations (ubuntu-latest)	4s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	32s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	34s
test-expectations (macos-latest)	14s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	34s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	32s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	30s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	34s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	36s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	35s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	34s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	35s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	34s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	34s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	35s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	34s
test (ubuntu-latest, 16.0, 0, 0, 1)	36s
test (macos-latest, 16.0, 0, 0, 0)	36s
success	1s
	11m 44s

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