Various small improvements of packer-related code and configuration

- 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!)