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/MANIFEST

@@ -524,7 +524,6 @@ xsp/Clipper.xsp
 xsp/Config.xsp
 xsp/ExPolygon.xsp
 xsp/ExPolygonCollection.xsp
-xsp/Extruder.xsp
 xsp/ExtrusionEntityCollection.xsp
 xsp/ExtrusionLoop.xsp
 xsp/ExtrusionMultiPath.xsp
@@ -534,7 +533,6 @@ xsp/Filler.xsp
 xsp/Flow.xsp
 xsp/GCode.xsp
 xsp/GCodeSender.xsp
-xsp/GCodeWriter.xsp
 xsp/Geometry.xsp
 xsp/GUI.xsp
 xsp/GUI_3DScene.xsp