Accept Java API interface types as arguments and return values from bound Go package functions and methods. Also, allow Go structs to extend Java classes and implement Java interfaces as well as override and implement methods. This is the third and final part of the implementation of the golang/go#16876 proposal. Fixes golang/go#16876 Change-Id: I6951dd87235553ce09abe5117a39a503466163c0 Reviewed-on: https://go-review.googlesource.com/28597 Reviewed-by: David Crawshaw <crawshaw@golang.org>