danger. major redo of core fdm slicer line connection algorithm. seems to handle non-manifold parts better. seems faster on torture tests. too good to be true?

src/geo/line.js

@@ -20,7 +20,7 @@
      * @constructor
      */
     function Line(p1, p2, key) {
-        if (!key) key = [p1.key, p2.key].join('-');
+        if (!key) key = [p1.key, p2.key].join(';');
         this.p1 = p1;
         this.p2 = p2;
         this.key = key;