Add custom list types which are suitable for Reclass's classes and applications arrays

[simu]

The UniqueList type corresponds to the Python Reclass Classes type (cf. https://github.com/kapicorp/reclass/blob/develop/reclass/datatypes/classes.py) and the RemovableList type corresponds to the Python Reclass Applications type (cf. https://github.com/kapicorp/reclass/blob/develop/reclass/datatypes/applications.py).

Notably, the RemovableList type incorporates the changes proposed to Python Reclass in kapicorp/reclass#9 which will address the longstanding issue where in some situations it's not possible to remove entries from the applications array.

Checklist

• The PR has a meaningful title. The title will be used to auto generate the changelog
• PR contains a single logical change (to build a better changelog).
• Update tests.
• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking, dependency, internal
  as they show up in the changelog

[bastjan]

Serde is cool 😊