Change `Callbacks` API. #1964

Job	Run time
quickchecking	18s
msrv	3m 20s
minimal	11s
docs	21s
rustfmt-clippy	27s
test-expectations (ubuntu-latest)	1m 3s
test-expectations (macos-latest)	2m 49s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	1m 4s
check-cfg	22s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	53s
test-book	12s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	58s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	53s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	49s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	56s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	54s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	56s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	1m 7s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	1m 5s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	1m 5s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	1m 5s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	1m 7s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	1m 7s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	1m 6s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	1m 8s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 6s
test (macos-latest, 9.0, 0, 0, 0)	1m 2s
success	1s
	27m 25s

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