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/tables/BitmapGlyphMetrics.py b/Lib/fontTools/ttLib/tables/BitmapGlyphMetrics.py
index 9cf4667..71514e1 100644
--- a/Lib/fontTools/ttLib/tables/BitmapGlyphMetrics.py
+++ b/Lib/fontTools/ttLib/tables/BitmapGlyphMetrics.py
@@ -1,6 +1,6 @@
 # Since bitmap glyph metrics are shared between EBLC and EBDT
 # this class gets its own python file.
-import sstruct
+from fontTools.misc import sstruct
 from types import TupleType
 from fontTools.misc.textTools import safeEval