`RepeatN`: implement `PeekingNext` #716

Job	Run time
msrv	39s
doc	14s
check	29s
check format	15s
check (--no-default-features)	1m 35s
test	1m 1s
check (--no-default-features --features use_alloc)	24s
check (--all-targets --all-features)	22s
All checks succeeded	1s
	5m 0s

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