Revert "Move unpack() from each implementation to __init__. (#286)"

This reverts commit da902f9c1d.

msgpack/fallback.py

@@ -100,6 +100,16 @@ def _get_data_from_buffer(obj):
     return view
 
 
+def unpack(stream, **kwargs):
+    """
+    Unpack an object from `stream`.
+
+    Raises `ExtraData` when `packed` contains extra bytes.
+    See :class:`Unpacker` for options.
+    """
+    data = stream.read()
+    return unpackb(data, **kwargs)
+
 
 def unpackb(packed, **kwargs):
     """