Ported the cooling changes from @alexrj: Don't slow down the external

perimeters if not necessary, don't take the bridging time into account
when slowing down the print.

Removed Extruder & GCodeWriter Perl bindings.
Improved Extruder for constness.
Refactored GCode::m_elapsed_time to struct ElapsedTime.

xs/xsp/typemap.xspt

@@ -99,9 +99,6 @@
 %typemap{MotionPlanner*};
 %typemap{Ref<MotionPlanner>}{simple};
 %typemap{Clone<MotionPlanner>}{simple};
-%typemap{GCodeWriter*};
-%typemap{Ref<GCodeWriter>}{simple};
-%typemap{Clone<GCodeWriter>}{simple};
 %typemap{GCodeSender*};
 %typemap{Ref<GCodeSender>}{simple};
 %typemap{Clone<GCodeSender>}{simple};
@@ -169,9 +166,6 @@
 %typemap{Polygons*};
 %typemap{TriangleMesh*};
 %typemap{TriangleMeshPtrs};
-%typemap{Extruder*};
-%typemap{Ref<Extruder>}{simple};
-%typemap{Clone<Extruder>}{simple};
 %typemap{Model*};
 %typemap{Ref<Model>}{simple};
 %typemap{Clone<Model>}{simple};