Add callback to declare functions/statics safe #2711

Job	Run time
check-cfg	24s
minimal	17s
docs	44s
msrv	49s
rustfmt-clippy	27s
quickchecking	18s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 39s
test-book	7s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 37s
test-expectations (ubuntu-latest)	41s
test-no-headers	27s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 40s
test-expectations (macos-latest)	41s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 51s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	2m 9s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	2m 10s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	2m 11s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	2m 8s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	4m 17s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	4m 21s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	4m 12s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	4m 2s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	4m 56s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	5m 27s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	4m 41s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	6m 2s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 32s
test (macos-latest, 16.0, 0, 0, 0)	2m 9s
success	1s
	1h 3m 0s

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