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-buffer-private.hh b/src/hb-buffer-private.hh
index dfcc45d..4c2a3e6 100644
--- a/src/hb-buffer-private.hh
+++ b/src/hb-buffer-private.hh
@@ -28,7 +28,7 @@
 #ifndef HB_BUFFER_PRIVATE_HH
 #define HB_BUFFER_PRIVATE_HH
 
-#include "hb-private.h"
+#include "hb-private.hh"
 #include "hb-buffer.h"
 #include "hb-unicode-private.hh"