perf(BREAKING): parse_to_value - borrow object keys with Cow<'a, str> to avoid per-property allocation

[dsherret]

We already borrow the values, so it's more consistent (and faster) to also borrow the keys instead of doing an additional allocation.

Bench	String keys	Cow keys	Speedup
package_json_value	23,361 ns	17,797 ns	−23.8% (1.31×)
key_heavy_json_value*	11,125,850 ns	8,154,100 ns	−26.7% (1.36×)
citm_catalog_json_large_value	29,799,430 ns	24,289,840 ns	−18.5% (1.23×)
tsconfig_json_value	2,329 ns	2,182 ns	−6.3% (1.07×)

This is unfortunately a breaking change, but I think it's more consistent and worth it.