feat: add custom attributes to `fn` items #2945

Job	Run time
rustfmt-clippy	12s
minimal	13s
docs	23s
msrv	23s
quickchecking	15s
check-cfg	21s
test-book	11s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	46s
test-expectations (ubuntu-latest)	48s
test-no-headers	20s
test-expectations (macos-latest)	42s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	43s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	43s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	43s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	44s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	47s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	50s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	47s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	47s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	45s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	47s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	48s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	46s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	45s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	46s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	48s
test (ubuntu-latest, 16.0, 0, 0, 1)	47s
test (macos-latest, 16.0, 0, 0, 0)	48s
success	1s
	17m 39s

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