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)