bind: generate Java constructors for every exported struct

A recent CL added Java constructors to generated classes that extends
or implements other Java classes and interfaces. Constructors for a
struct S are Go functions on the form

func NewS...(...) *S

If no such constructors exists, a default empty constructor is
generated.

Expand that to cover every exported Go struct.

Fixes golang/go#17086

Change-Id: I910aba13d5884c3f67c946c62a8ac4a3db8e2ea7
Reviewed-on: https://go-review.googlesource.com/29710
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/testdata/structs.java.golden

@@ -19,6 +19,10 @@ public final class S implements Seq.Proxy {
     
     S(Seq.Ref ref) { this.ref = ref; }
     
+    public S() { this.ref = __New(); }
+    
+    private static native Seq.Ref __New();
+    
     public final native double getX();
     public final native void setX(double v);
     
@@ -79,6 +83,10 @@ public final class S2 implements Seq.Proxy, I {
     
     S2(Seq.Ref ref) { this.ref = ref; }
     
+    public S2() { this.ref = __New(); }
+    
+    private static native Seq.Ref __New();
+    
     public native void m();
     public native String string();
     @Override public boolean equals(Object o) {