Common glue code between Rust and JNI, used in autogenerated jni-bindgen glue code.
See also the [Android JNI tips](https://developer.android.com/training/articles/perf-jni) documentation as well as the
[Java Native Interface Specification](https://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/jniTOC.html).