Change `Callbacks` API. #1958

Job	Run time
msrv	4m 2s
docs	20s
minimal	20s
check-cfg	24s
quickchecking	26s
rustfmt-clippy	28s
test (ubuntu-latest, 9.0, 1, 0, 0, 0, 0)	1m 42s
test-book	6s
test (ubuntu-latest, 9.0, 1, 0, 0, 1, 0)	1m 40s
test (ubuntu-latest, 9.0, 1, 0, 1, 0, 0)	1m 43s
test-expectations (ubuntu-latest)	1m 1s
test (ubuntu-latest, 9.0, 1, 0, 1, 1, 0)	1m 40s
test-expectations (macos-latest)	4m 25s
test (ubuntu-latest, 9.0, 1, 1, 0, 0, 0)	1m 41s
test (ubuntu-latest, 9.0, 1, 1, 0, 1, 0)	1m 42s
test (ubuntu-latest, 9.0, 1, 1, 1, 0, 0)	1m 41s
test (ubuntu-latest, 9.0, 1, 1, 1, 1, 0)	1m 42s
test (ubuntu-latest, 16.0, 1, 0, 0, 0, 0)	1m 41s
test (ubuntu-latest, 16.0, 1, 0, 0, 1, 0)	1m 43s
test (ubuntu-latest, 16.0, 1, 0, 1, 0, 0)	1m 42s
test (ubuntu-latest, 16.0, 1, 0, 1, 1, 0)	1m 43s
test (ubuntu-latest, 16.0, 1, 1, 0, 0, 0)	1m 43s
test (ubuntu-latest, 16.0, 1, 1, 0, 1, 0)	1m 42s
test (ubuntu-latest, 16.0, 1, 1, 1, 0, 0)	1m 39s
test (ubuntu-latest, 16.0, 1, 1, 1, 1, 0)	1m 42s
test (ubuntu-latest, 16.0, 0, 0, 1)	1m 42s
test (macos-latest, 9.0, 0, 0, 0)	1m 26s
success	1s
	41m 47s

Provide feedback