Add macro `iunpack` #685

Job	Run time
check format	7s
msrv	36s
doc	5s
check	6s
test	52s
check (--no-default-features)	9s
check (--no-default-features --features use_alloc)	12s
check (--all-targets --all-features)	20s
All checks succeeded	1s
	2m 28s

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