feat: add custom attributes to `fn` items #2944

Job	Run time
check-cfg	19s
msrv	49s
docs	22s
minimal	18s
quickchecking	17s
rustfmt-clippy	14s
test-book	14s
test-expectations (ubuntu-latest)	40s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 37s
test-expectations (macos-latest)	1m 5s
test-no-headers	28s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 26s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 23s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 24s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	1m 38s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	1m 37s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	1m 36s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	1m 36s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	1m 39s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	1m 38s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	1m 38s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	1m 38s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	1m 38s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	1m 38s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	1m 38s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	1m 35s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 37s
test (macos-latest, 16.0, 0, 0, 0)	1m 41s
success	1s
	33m 24s

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