Add UniqueHandle non-const accessors, ensure all by reference. (#123)

* Add UniqueHandle non-const accessors, ensure all by reference.

- Add `Type * operator->()`
- Fix `Type get() const` to `const Type & get() const`
- Add `Type & get()`

* Add support for structure pointer chains was missing some generator code.

* Update vulkan.hpp with changes.

vulkan/vulkan.hpp

@@ -361,13 +361,28 @@ namespace vk
     {
       return &m_value;
     }
-
+    
+    Type * operator->()
+    {
+      return &m_value;
+    }
+    
     Type const& operator*() const
     {
       return m_value;
     }
 
-    Type get() const
+    Type & operator*()
+    {
+      return m_value;
+    }
+
+    const Type & get() const
+    {
+      return m_value;
+    }
+    
+    Type & get()
     {
       return m_value;
     }