Improved tests for simple 1-1 opposites
Improved Opposite annotation so that it passes the tests from SHA: 4181486

To achieve this, 4 methods are created in the generated code of the
class containing an @Opposite attribute called "attr":
    - A simple getter "attr()" that returns its current value
    - A proxy setter "attr(obj)" that propagates the changes
    - A resetter "__k3_attr_reset() that nullify its value
    - A simple setter "__k3_attr_set(obj)" that sets its value