eo: memory waste at mixin class elaboration

eo: memory waste at mixin class elaboration

@fix

mixins data offsets are stored in Eo_Extension_Data_Offset[],
if the constructed class is a mixin, do not reserve space for its
private data, the class is in mixins list and will be handled at
Eo_Extension_Data_Offset computation.

see  _eo_data_scope_get(...) for private data retrieval