Change `Callbacks` API. #2026

Job	Run time
check-cfg	26s
docs	32s
minimal	26s
msrv	1m 47s
quickchecking	25s
rustfmt-clippy	31s
test-book	7s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	2m 15s
test-expectations (ubuntu-latest)	55s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	2m 22s
test-expectations (macos-latest)	2m 13s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	1m 54s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	1m 54s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	2m 41s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	2m 38s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	2m 51s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	3m 4s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	2m 59s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	3m 28s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	2m 35s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	3m 25s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	3m 28s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	3m 23s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	3m 1s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	3m 31s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 40s
test (macos-latest, 9.0, 0, 0, 0)	1m 29s
success	3s
	57m 3s

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