Remove not useful detachable mark in eg_item.e
Add missing redefinition clause and redefine `is_equal' since `hash_code' is redefined.
Improve implementation of eg_quad_tree.valid_tree in order to remove a check clause.
Prepare immutability of eg_vector2d.e
Use the creation procedure `EG_VECTOR2D.make' since `EG_VECTOR2D.set' is obsolete