Add support for unsafe extern blocks #2608

Job	Run time
rustfmt-clippy	28s
minimal	18s
msrv	49s
quickchecking	22s
test-expectations (ubuntu-latest)	55s
test-expectations (macos-12)	1m 9s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	2m 7s
check-cfg	22s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	1m 44s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	1m 46s
docs	25s
test-book	15s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	1m 55s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	2m 15s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	2m 29s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	2m 17s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	2m 13s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	4m 15s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	4m 26s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	4m 14s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	4m 6s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	4m 51s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	4m 57s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	4m 47s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	4m 57s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 17s
test (macos-12, 9.0, 0, 0, 0)	1m 33s
success	1s
	1h 2m 13s

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