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-unicode.cc b/src/hb-unicode.cc
index 9295be7..282a9b6 100644
--- a/src/hb-unicode.cc
+++ b/src/hb-unicode.cc
@@ -28,7 +28,7 @@
  * Google Author(s): Behdad Esfahbod
  */
 
-#include "hb-private.h"
+#include "hb-private.hh"
 
 #include "hb-unicode-private.hh"