Refactor System Fields

[max-moser]

AFAICS, it doesn't seem like the "public" methods (e.g. obj()) are part of an interface for system fields that's being relied on anywhere.
As per discussion, it could be beneficial to slightly rename the methods in system fields, to emphasize to the developers that they are Python Descriptors.

Another possible means for improvement would be the __set_name__(...) magic method that could remove the necessity for explicitly setting the system field's backing property in many cases.