Rename all private sources and headers to C++ files

So we can liberally use the simple features of C++ that parts of the
codebase is already using.
diff --git a/src/hb-ot-shape-complex-private.hh b/src/hb-ot-shape-complex-private.hh
index 8147945..8225b46 100644
--- a/src/hb-ot-shape-complex-private.hh
+++ b/src/hb-ot-shape-complex-private.hh
@@ -27,7 +27,7 @@
 #ifndef HB_OT_SHAPE_COMPLEX_PRIVATE_HH
 #define HB_OT_SHAPE_COMPLEX_PRIVATE_HH
 
-#include "hb-private.h"
+#include "hb-private.hh"
 
 #include "hb-ot-shape-private.hh"