Move extrusion path gcode generation to C++ for speed.

xs/xsp/ExtrusionPath.xsp

@@ -27,6 +27,9 @@
     bool is_perimeter();
     bool is_fill();
     bool is_bridge();
+    std::string gcode(SV* extruder, double e, double F,
+        double xofs, double yofs, std::string extrusion_axis,
+        std::string gcode_line_suffix);
 %{
 
 ExtrusionPath*