perf(serde): borrow object keys during deserialization to avoid per-key allocation

[dsherret]

Bench	Before	After	Result
key_heavy_json_serde_struct	7,987,740 ns	5,637,590 ns	−29.4% (1.42×)
package_json_serde_value	26,536 ns	26,572 ns	flat (no regression)
tsconfig_json_serde_value	2,356 ns	2,347 ns	flat (no regression)
citm_catalog_json_large_serde_value	27,578,010 ns	28,703,940 ns	flat (within noise)