feat: add custom attributes to `fn` items #2941

Job	Run time
rustfmt-clippy	30s
msrv	53s
minimal	14s
check-cfg	21s
docs	17s
quickchecking	13s
test-book	6s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 27s
test-expectations (ubuntu-latest)	44s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 25s
test-no-headers	27s
test-expectations (macos-latest)	49s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 24s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 31s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	1m 38s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	1m 38s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	1m 38s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	1m 39s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	1m 40s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	1m 39s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	1m 39s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	1m 39s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	1m 40s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	1m 40s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	1m 40s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	1m 38s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 40s
test (macos-latest, 16.0, 0, 0, 0)	1m 41s
success	1s
	33m 31s

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