provide GenericAlignedStruct that auto Pads itself to keep alignment.

[erickvermot]

While implementing Match and ActionSetField classes (#348), padding to keep alignment of variable size structs was implemented.
I suggest we place the custom get_size and pack methods in a new base class called GenericAlignedStruct to be used whenever needed, avoiding reimplementation and bugs.
(the methods implementation can be found in the PR #392.)