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/prototest/test.golden

@@ -9,14 +9,10 @@ enum ProtoEnum : int {
   BAR = 5,
 }
 
-namespace _proto._test;
-
 table ImportedMessage {
   a:int;
 }
 
-namespace _proto._test;
-
 /// 2nd table doc comment with
 /// many lines.
 table ProtoMessage {