vk-layer-generate.py: put layer functions into a namespace

Move everything into namespace object_tracker and unique_objects
respectively.  I had to add some wrappers to make everything work.
diff --git a/layers/unique_objects.h b/layers/unique_objects.h
index 6c2c2e3..0da90a1 100644
--- a/layers/unique_objects.h
+++ b/layers/unique_objects.h
@@ -39,6 +39,8 @@
 #include "vk_safe_struct.h"
 #include "vk_layer_utils.h"
 
+namespace unique_objects {
+
 // All increments must be guarded by global_lock
 static uint64_t global_unique_id = 1;
 
@@ -393,3 +395,5 @@
     }
     return result;
 }
+
+} // namespace unique_objects