Add `peek_mut` and `peek_nth_mut` to `PeekNth` #520

Job	Run time
msrv	1m 34s
test	1m 35s
check format	20s
check	23s
check (--no-default-features)	19s
check (--no-default-features --features use_alloc)	20s
check (--all-targets --all-features)	34s
All checks succeeded	1s
	5m 6s

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