feat: add custom attributes to `fn` items #2807

Job	Run time
minimal	19s
docs	24s
check-cfg	19s
msrv	48s
quickchecking	23s
rustfmt-clippy	12s
test-book	6s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 43s
test-expectations (ubuntu-latest)	27s
test-no-headers	27s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 49s
test-expectations (macos-latest)	43s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 47s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 59s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	2m 42s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	2m 20s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	2m 26s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	2m 19s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	4m 40s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	4m 30s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	4m 57s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	4m 55s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	5m 25s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	4m 57s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	4m 58s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	19m 40s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 42s
test (macos-latest, 16.0, 0, 0, 0)	2m 12s
success	1s
	1h 20m 10s

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