feat: add custom attributes to `fn` items #2931

Job	Run time
minimal	16s
check-cfg	23s
docs	21s
msrv	57s
quickchecking	20s
rustfmt-clippy	13s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 36s
test-book	19s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 30s
test-expectations (ubuntu-latest)	41s
test-no-headers	37s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 27s
test-expectations (macos-latest)	54s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 39s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	1m 47s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	1m 47s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	1m 47s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	1m 47s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	1m 49s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	1m 49s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	1m 49s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	1m 48s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	1m 49s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	1m 48s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	1m 49s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	1m 48s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 49s
test (macos-latest, 16.0, 0, 0, 0)	1m 51s
success	1s
	36m 31s

Provide feedback