devalue: Sparse arrays emitted by uneval cause eager allocation when evaluated

Summary

Evaluating legitimate `uneval` output for a sparse array can allocate memory proportional to its declared length. A tiny serialized value can therefore cause large memory allocation in a consuming browser/runtime. This occurs during evaluation of generated code, not in default `parse` sparse-array construction. You would only be affected by this if you were serializing very large sparse arrays and then evaluating the results. In the general use case for `uneval` of sending data to the client, the worst that could happen is the browser tab running out of memory.