eina_mono: replace EinaAccessor and EinaIerator with IEnumerable

Summary:
Eina.Accessor<T> => System.Collections.IEnumerable<T>
Eina.Iterator<T> => System.Collections.IEnumerable<T>

Unit test will work with D10879.

ref T8486

Test Plan: meson build -Dbindings=mono,cxx -Dmono-beta=true

Reviewers: lauromoura

Reviewed By: lauromoura

Subscribers: cedric, reviewers, committers

Tags: efl

Maniphest Tasks: T8486

Differential Revision: https://phab.enlightenment.org/D10878