bind,cmd,internal: generate reverse bindings for exported Go structs

Before this CL, the type of the implicit "this" parameter to Java methods
implemented in Go could only be a super class of the generated Java
class. For example, the following GoRunnable type is an implementation of
the Java interface java.lang.Runnable with a toString method:

package somepkg

import "Java/java/lang"

type GoRunnable struct {
    lang.Runnable
}

func (r *GoRunnable) ToString(this lang.Runnable) string {
    ...
}

The "this" parameter is implicit in the sense that the reverse generator
automatically fills it with a reference to the Java instance of
GoRunnable.

Note that "this" has the type Java/java/lang.Runnable, not
Java/go/somepkg.GoRunnable, which renders it impossible to call Java
methods and functions that expect GoRunnable. The most practical example
of this is the Android databinding libraries.

This CL changes the implicit this parameter to always match the exact
type. In the example, the toString implementation becomes:

import gopkg "Java/go/somepkg"

func (r *GoRunnable) ToString(this gopkg.GoRunnable) string {
    ...
}

One strategy would be to simply treat the generated Java classes
(GoRunnable in our example) as any other Java class and import it
through javap. However, since the Java classes are generated after
importing, this present a chicken-and-egg problem.

Instead, use the newly added support for structs with embedded prefixed types
and synthesize class descriptors for every exported Go struct type.

Change-Id: Ic5ce4a151312bd89f91798ed4088c9959225b448
Reviewed-on: https://go-review.googlesource.com/34776
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/testdata/java.java.h.golden

@@ -62,14 +62,10 @@ extern jmethodID proxy_class_java_F_cons;
 extern jclass proxy_class_java_O;
 extern jmethodID proxy_class_java_O_cons;
 
-// skipped method O.Super with unsupported parameter or return types
-
 extern jclass proxy_class_java_R;
 extern jmethodID proxy_class_java_R_cons;
 
 extern jclass proxy_class_java_S;
 extern jmethodID proxy_class_java_S_cons;
 
-// skipped method S.Super with unsupported parameter or return types
-
 #endif