Get code generator plugin to a decently working state.

Still a few outstanding performance issues, and imports inside .capnp
files may not be working

examples/addressbook.capnp

@@ -18,7 +18,6 @@ struct Person {
       work @2;
     }
   }
-
   employment :union {
     unemployed @4 :Void;
     employer @5 :Text;
@@ -26,6 +25,13 @@ struct Person {
     selfEmployed @7 :Void;
     # We assume that a person is only one of these.
   }
+
+  testGroup :group {
+    field1 @8 :UInt32;
+    field2 @9 :UInt32;
+    field3 @10 :UInt32;
+  }
+  extraData @11 :Data;
 }
 
 struct AddressBook {