Add support for unsafe extern blocks #2628

Job	Run time
check-cfg	20s
minimal	23s
docs	28s
msrv	56s
quickchecking	21s
rustfmt-clippy	29s
test-book	6s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	1m 44s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	1m 54s
test-expectations (ubuntu-latest)	51s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	1m 48s
test-expectations (macos-12)	1m 3s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	1m 55s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	2m 17s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	2m 18s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	2m 33s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	2m 19s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	4m 24s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	4m 9s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	4m 6s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	4m 59s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	4m 53s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	4m 55s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	5m 40s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	4m 50s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 47s
test (macos-12, 9.0, 0, 0, 0)	1m 35s
success	1s
	1h 3m 4s

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