feat: add custom attributes to `fn` items #2918

Job	Run time
minimal	12s
rustfmt-clippy	26s
msrv	51s
quickchecking	14s
docs	19s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 27s
test-expectations (ubuntu-latest)	59s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 24s
test-expectations (macos-latest)	54s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 25s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 26s
check-cfg	22s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	1m 38s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	1m 39s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	1m 39s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	1m 39s
test-book	10s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	1m 40s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	1m 41s
test-no-headers	29s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	1m 39s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	1m 40s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	1m 40s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	1m 39s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	1m 39s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	1m 39s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 41s
test (macos-latest, 16.0, 0, 0, 0)	1m 43s
success	1s
	33m 55s

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