Create Packable interface class

Create Packable interface class

Packer class model used C-style function pointers and a standalone
object to perform C-style trampoline for function/method calls.

C++ virtual methods offer to inline all that directly in the data store
objects and enforces type safety on the child object methods instead of
forcing manual type casting on developers.

Re-implement Packer as a wrapper class providing the Packable interface
for backward compatibility with Packer* code. Future code should inherit
objects directly from Packable and implement the interface.