Cleaned up namespace handling.

A Namespace object is now guaranteed unique.
This cleaned up some old workarounds and latent bugs.

Change-Id: Ic3f12d89947871b03b2c449ba51b3186f953adde
Tested: on Linux.
Bug: 21336857

tests/monster_test.fbs

@@ -2,6 +2,10 @@
 
 include "include_test1.fbs";
 
+namespace MyGame;
+
+table InParentNamespace {}
+
 namespace MyGame.Example2;
 
 table Monster {}  // Test having same name as below, but in different namespace.
@@ -77,6 +81,7 @@ table Monster {
   flex:[ubyte] (id:30, flexbuffer);
   vector_of_longs:[long] (id:32);
   vector_of_doubles:[double] (id:33);
+  parent_namespace_test:InParentNamespace (id:34);
 }
 
 table TypeAliases {