bind: support reference types as struct fields.

Reference types here mean pointers to struct types and
exported interface types. Previously, gobind generated invalid
go/java sources.

Change-Id: Icd666ebbb8edb70c613311798d602fd88cb4479c
Reviewed-on: https://go-review.googlesource.com/9997
Reviewed-by: Alan Donovan <adonovan@google.com>

bind/testdata/structs.java.golden

@@ -42,6 +42,7 @@ public abstract class Structs {
             in.writeFloat64(v);
             Seq.send(DESCRIPTOR, FIELD_X_SET, in, out);
         }
+        
         public double getY() {
             Seq in = new Seq();
             Seq out = new Seq();