Add csv exporter

[patacca]

Fix https://github.com/quarkslab/qbindiff/issues/30 by adding a csv exporter.

This PR will also come with few changes:

• Introduce the GenericNode abstract class to define what a "node" in a GenericGraph is. This will replace the previous Item type that was just an alias over Any.
• Some changes in the API of the Mapping class.
• Avoid multiple inheritance with classes with different metaclasses on Program. This is related to https://github.com/python/cpython/issues/50246
• Clean code and better typing support

Still missing:

• [x] Add documentation in diffing-portal about the new exporter
• [x] Make the csv exporter as the default one in the CLI
• [x] Change the README.md help description

[patacca]

PR ready for review

[RobinDavid]

Sounds great for me!