feat: add custom attributes to `fn` items #2805

Job	Run time
quickchecking	20s
msrv	47s
docs	26s
minimal	19s
rustfmt-clippy	29s
test-expectations (ubuntu-latest)	16s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 50s
test-expectations (macos-latest)	13s
check-cfg	22s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 55s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 41s
test-book	7s
test-no-headers	33s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 40s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	1m 57s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	1m 55s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	1m 55s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	1m 56s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	1m 57s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	1m 56s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	1m 57s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	1m 57s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	1m 57s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	1m 58s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	1m 58s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	1m 56s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 57s
test (macos-latest, 16.0, 0, 0, 0)	1m 58s
success	1s
	38m 13s

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