feat: add custom attributes to `fn` items #2935

Job	Run time
docs	9s
minimal	33s
check-cfg	20s
msrv	23s
quickchecking	17s
rustfmt-clippy	12s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	43s
test-book	6s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	43s
test-expectations (ubuntu-latest)	47s
test-no-headers	17s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	52s
test-expectations (macos-latest)	49s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	50s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	45s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	42s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	41s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	43s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	44s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	45s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	44s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	42s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	43s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	42s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	42s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	44s
test (ubuntu-latest, 16.0, 0, 0, 1)	44s
test (macos-latest, 16.0, 0, 0, 0)	44s
success	1s
	17m 7s

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