packer/unpacker cleanup / consolidation

• Remove the need for upfront_decoder.bind( source = ... ): compute the relevant argument from the requested location in _get_unpacked and pass it explicitly
• Remove ability to pack using deprecated, old-style 32-bit references which are known to overflow and lead to data corruption - only keep the ability to pack 64-bit references. Throw an exception if such an packing version still requested (note: the unpacking is left as-is)
• Improve implementation of packing and unpacking of references
• Unify setters in event model classes for SmartRef and pointer: remove redundant overloads (which could be ambiguous, and require additional casts!)

Sits on top of !4494 (merged)

Goes together with Rec!3836 (merged), Moore!3117 (merged), DaVinci!1056 (merged)

Event/EventBase/include/Event/FunctorDefaults.h

@@ -51,11 +51,16 @@ namespace LHCb::Event {
                                                Ret>;
 
     template <typename T>
-    void check_null_pointer( T* ptr, std::string_view sv ) {
+    void check_null_pointer( T const* ptr, std::string_view sv ) {
       if ( !ptr )
         throw GaudiException( "null pointer not allowed at this point", std::string{sv}, StatusCode::FAILURE );
     }
 
+    template <typename T>
+    void check_null_pointer( SmartRef<T> const& ref, std::string_view sv ) {
+      return check_null_pointer( ref.target(), sv );
+    }
+
   } // namespace details
 
   /* when we have different accessor names, we use ADL, with as default this indirection layer */