feat: add custom attributes to `fn` items #2810

Job	Run time
rustfmt-clippy	29s
docs	24s
minimal	19s
msrv	49s
quickchecking	23s
check-cfg	22s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 45s
test-book	22s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 54s
test-expectations (ubuntu-latest)	16s
test-no-headers	25s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 45s
test-expectations (macos-latest)	13s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 44s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	2m 34s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	2m 30s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	2m 24s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	2m 35s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	4m 31s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	9m 14s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	9m 52s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	9m 48s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	5m 2s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	5m 14s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	5m 4s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	9m 37s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 32s
test (macos-latest, 16.0, 0, 0, 0)	2m 12s
success	1s
	1h 24m 20s

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