`PutBack::put_back` now returns the old value #785

Job	Run time
check format	8s
doc	14s
msrv	54s
check	18s
test	51s
semver-checks	2m 20s
check (--no-default-features)	16s
check (--no-default-features --features use_alloc)	6s
check (--all-targets --all-features)	22s
All checks succeeded	1s
	5m 30s

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