First implementation of simplex mesh.
Major restructuring.

        - geomety-related traits are now on their own folder, and implementations for each geometry
          are on files named trait_geometrytype.rs.
        - the collision dispatcher is now based on trait-object and dynamic cast. It is slightly
          faster than the old dispatcher based on enums.
        - tests are re-enabled.
        - add the support of triangle meshes.

For now, ray-casting based continuous collision detection is broken.