make GenericStruct and GenericType derivatives reuse pack and get_size code

[erickvermot]

Many GenericType and GenericStruct derivatives reimplement pack(value) and get_size(value), and each one rewrites the code that deals with the value parameter, as well as the docstring.

making GenericType and GenericStruct offer a pack(self, value) that deal with the parameter and offers the docstring, and internally calls the instance's _pack(self) method makes it possible to the derivatives to rewrite _pack and avoid having to rewrite the code to deal with the parameter and with the docstring, reducing a lot of redundant code.

This was implemented in the match fix PR (https://github.com/kytos/python-openflow/pull/392/commits) (mostly inside the commit a4c1f05), but I was told to close the PR and open a separate issue for that.

+ If this is implemented, then the issue (#196) becomes trivial.