layers: Fix compilation on 32bit platforms

Merge Request 57
     Remove use of nullptr for handles
     Modify maps to use uint64_t
     Modify overloaded functions to have different signatures

Conflicts:
	vk-layer-generate.py
diff --git a/layers/threading.h b/layers/threading.h
index a6fd869..cbff088 100644
--- a/layers/threading.h
+++ b/layers/threading.h
@@ -42,7 +42,7 @@
 
     layer_data() :
         report_data(nullptr),
-        logging_callback(nullptr)
+        logging_callback(VK_NULL_HANDLE)
     {};
 };