`Diff`: implement `Debug` and `Clone` #693

Job	Run time
msrv	36s
doc	8s
check	13s
check format	6s
check (--no-default-features)	7s
test	47s
check (--no-default-features --features use_alloc)	9s
check (--all-targets --all-features)	19s
All checks succeeded	1s
	2m 26s

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