Move sstruct under fontTools.misc

Our footprint in the Python module namespace is all under
fontTools now.  User code importing sstruct should be updated
to say "from fontTools.misc import sstruct".
diff --git a/Lib/fontTools/ttLib/sfnt.py b/Lib/fontTools/ttLib/sfnt.py
index 43bf78b..82e3e04 100644
--- a/Lib/fontTools/ttLib/sfnt.py
+++ b/Lib/fontTools/ttLib/sfnt.py
@@ -13,7 +13,8 @@
 """
 
 import sys
-import struct, sstruct
+import struct
+from fontTools.misc import sstruct
 import os