feat: add custom attributes to `fn` items #2798

Job	Run time
minimal	12s
msrv	1m 4s
rustfmt-clippy	29s
check-cfg	20s
docs	27s
quickchecking	20s
test-book	10s
test-expectations (ubuntu-latest)	29s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 41s
test-expectations (macos-latest)	46s
test-no-headers	27s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 52s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 43s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 45s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	2m 11s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	2m 18s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	2m 32s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	2m 12s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	20m 1s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	20m 3s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	19m 18s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	19m 13s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	10m 3s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	10m 11s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	20m 7s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	4m 48s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 56s
test (macos-latest, 16.0, 0, 0, 0)	2m 5s
success	1s
	2h 29m 44s

Provide feedback