Refactored corners to position motor based on motor_shaft_xoffset; changed rail holder to use a rectangular block, added slots for belt tension adjustment

lib.scad

@@ -2,4 +2,13 @@ module bom_item(printable = true, label = "") {
   print_label = printable ? " print " : " stock ";
   item_label = label == "" ? parent_module(1) : label;
   echo(str("BOM:", print_label, item_label));
+}
+
+module slotify(vector) {
+  for (child = [0:$children-1]) {
+    hull() {
+      children(child);
+      translate(vector) children(child);
+    }
+  }
 }