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.

src/hb-shape.cc

diff --git a/src/hb-shape.cc b/src/hb-shape.cc
index 9d5d418..66d951b 100644
--- a/src/hb-shape.cc
+++ b/src/hb-shape.cc
@@ -24,7 +24,7 @@
  * Red Hat Author(s): Behdad Esfahbod
  */
 
-#include "hb-private.h"
+#include "hb-private.hh"
 
 #include "hb-shape.h"