serialize: Deprecate begin_ptr / end_ptr

Implement `begin_ptr` and `end_ptr` in terms of C++11 code,
and add a comment that they are deprecated.

Follow-up to developer notes update in 654a211622.

src/prevector.h

@@ -476,7 +476,7 @@ public:
         }
     }
 
-    value_type* data() noexcept {
+    value_type* data() {
         return item_ptr(0);
     }