feat: add custom attributes to `fn` items #2890

Job	Run time
check-cfg	27s
docs	19s
minimal	53s
msrv	28s
rustfmt-clippy	22s
quickchecking	36s
test-book	49s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 11s
test-expectations (ubuntu-latest)	1m 35s
test-no-headers	53s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 10s
test-expectations (macos-latest)	48s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 15s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	57s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	1m 13s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	1m 14s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	1m 4s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	1m 12s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	1m 2s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	1m 3s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	1m 3s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	56s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	1m 2s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	1m 1s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	1m 1s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	1m 1s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 3s
test (macos-latest, 16.0, 0, 0, 0)	1m 5s
success	1s
	26m 44s

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