Structs

It'd be useful to support some kind of `@KneeStruct` annotation to mark classes that we want to pass-by-value.

- Could replace `@KneeEnum` and `@KneeObject`, which follow the same semantics (e.g. a JVM enum doesn't hold a reference to the native enum)
- Could apply to `value class`, something you can't currently send
- Could apply to `data class` and, if they ever show up in the language, multiple-values value classes

The codec implementation for structs could use a `jobjectArray` under the hood.