devalue affected by CPU and memory amplification from sparse arrays

Summary

Under certain circumstances, serializing sparse arrays using `uneval` or `stringify` could cause CPU and/or memory exhaustion. When this occurs on the server, it results in a DoS. This is extremely difficult to take advantage of in practice, as an attacker would have to manage to create a sparse array on the server — which is impossible in every mainstream wire format — and then that sparse array would have to be run through `uneval` or `stringify`.