Add `peek_mut` and `peek_nth_mut` to `PeekNth` #343

Job	Run time
test	1m 22s
check (msrv)	1m 7s
check (msrv, --no-default-features)	1m 18s
check (msrv, --no-default-features --features use_alloc)	1m 4s
check (stable)	8s
check (stable, --no-default-features)	5s
check (stable, --no-default-features --features use_alloc)	14s
check (stable, --all-targets --all-features)	26s
bors build finished	1s
	5m 45s

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