New function [get_cvalue_model_result] returns the error cases without
conversion.

Nonterm: uses List.iter and Eva.Results.callstacks instead of iter_callstacks.

Removes [get_kinstr_state].
Uses these new functions in Results, to be more consistent.

Adds function [as_zone_result] that returns a zone result, without converting
error cases.

Changes the Inout plugin accordingly.