Replace Packer object API with Packable API

Replace Packer object API with Packable API

Majority of thost patch is symbol renaming to unify the
class method names to the Packable API names.

There is effectively no logical change in this patch
despite appearances because it replaces the Packer object
which provides methods which are just wrappers pointing
to static functions which are in turn wrappers pointing
to storage buffer object methods. With direct calls to
those storage object methods (renamed).

We can now interchangebly use MemBuf or StoreEntry objects
with the packInto(Packable *) functions. Or any other
object which inherits and implements the Packable API.

We also gain 0.1% in performance (+2 RPS) by avoiding the
layers of wrapper funcions and Packer object allocate /
deallocate cycles.