Make a few more tests pass with the new I/O library.

Fix the truncate() semantics -- it should not affect the current position.
Switch wave.py/chunk.py to struct.unpack_from() to support bytes.
Don't use writelines() on binary files (test_fileinput.py).

Lib/io.py

@@ -551,8 +551,6 @@ class _MemoryIOMixin(BufferedIOBase):
     def truncate(self, pos=None):
         if pos is None:
             pos = self._pos
-        else:
-            self._pos = max(0, pos)
         del self._buffer[pos:]
         return pos