Add missing `must_use` attributes on adaptors #575

Job	Run time
test	1m 51s
check format	8s
msrv	1m 57s
check	26s
check (--no-default-features)	27s
check (--no-default-features --features use_alloc)	7s
check (--all-targets --all-features)	24s
All checks succeeded	3s
	5m 23s

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