feat: add custom attributes to `fn` items #2940

Job	Run time
docs	18s
minimal	17s
msrv	52s
quickchecking	13s
check-cfg	20s
rustfmt-clippy	12s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 34s
test-book	8s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 26s
test-expectations (ubuntu-latest)	44s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 35s
test-expectations (macos-latest)	42s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 35s
test-no-headers	23s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	1m 43s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	1m 42s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	1m 41s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	1m 41s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	1m 44s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	1m 42s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	1m 44s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	1m 43s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	1m 41s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	1m 43s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	1m 42s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	1m 43s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 41s
test (macos-latest, 16.0, 0, 0, 0)	1m 44s
success	1s
	34m 14s

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