Remove cast to bool

As Greg suggested, the cast to bool is no longer needed if we can cast
to an int, so we removed it. This commit also adds the uint64_t cast to
the pseudo class definition for BufferPointers.

proposals/0010-vk-buffer-ref.md

@@ -86,6 +86,7 @@ class vk::BufferPointer {
     vk::BufferPointer& operator=(const vk::BufferPointer&);
     vk::BufferPointer(const uint64_t);
     S& Get() const;
+    operator uint64_t() const;
 }
 ```
 
@@ -105,10 +106,8 @@ This new type will have the following operations
     vk::BufferPointer<DstType, DstAlign> only if SrcType is a type derived from
     DstType. vk::reinterpret_pointer_cast<T, A> allows casting for all other
     BufferPointer types. For both casts, DstAlign <= SrcAlign must be true.
-*   A buffer pointer can be constructed from a uint64_t u using the constructor
+*   A buffer pointer can be constructed from a uint64_t using the constructor
     syntax vk::BufferPointer<T,A>(u).
-*   A buffer pointer can be cast to a bool. If so, it returns FALSE if the
-    pointer is null, TRUE otherwise.
 *   A buffer pointer can be cast to a uint64_t. The cast will return the 64-bit
     address that the pointer points to.