eina: add eina_multi_iterator_new.

eina: add eina_multi_iterator_new.

This will return an Iterator that will walk over a serie of iterator.
This technicaly take ownership of the iterator it is walking over and
destroy as it goes and doesn't need them anymore.

Reviewed-by: Vitor Sousa da Silva <vitorsousa@expertisesolutions.com.br>
Differential Revision: https://phab.enlightenment.org/D7286