json-iterator/go ? reverse-engineered prompt

Build me a Go package that works like a faster replacement for the standard encoding/json library.

I want to be able to swap it in with almost no code changes, so Marshal, Unmarshal, and the usual JSON handling should feel the same as the Go standard library. Please make it fast, reliable, and compatible with normal Go structs, maps, slices, numbers, strings, and nested objects. It should also handle common edge cases cleanly, like null values, invalid input, and custom marshaling behavior.

If it helps, look up the current docs online and match the expected behavior as closely as possible. I’d also like good tests that compare it against the standard library so I can trust it as a drop in replacement.