Rename/rejig so as to have just a single Impl and then R2 and R3 stretcher classes

rubberband/RubberBandStretcher.h

@@ -81,7 +81,6 @@
 
 namespace RubberBand
 {
-class R3StretcherImpl;//!!!
 
 class RUBBERBAND_DLLEXPORT
 RubberBandStretcher
@@ -807,7 +806,9 @@ public:
 protected:
     class Impl;
     Impl *m_d;
-    R3StretcherImpl *m_r3d;
+
+    RubberBandStretcher(const RubberBandStretcher &) =delete;
+    RubberBandStretcher &operator=(const RubberBandStretcher &) =delete;
 };
 
 }