Speed improvement for vnf_vertex_array by changing deduplicate

testing.  Add "min_edge" style to vnf vertex array.
eliminate _skin_core, replace with vnf_vertex_array, add style option
to all modules/functions.
fix bug in path_normals (not normalized)

paths.scad

@@ -380,7 +380,7 @@ function path_normals(path, tangents, closed=false) =
         dim == 2 ? [tangents[i].y,-tangents[i].x]
                  : let(v=cross(cross(pts[1]-pts[0], pts[2]-pts[0]),tangents[i]))
                    assert(norm(v)>EPSILON, "3D path contains collinear points")
-                   v
+                   unit(v)
     ];