Use (new) buffer protocol in Pickle.decode on Python 2 (#150)

* Use (new) buffer protocol in `Pickle.decode`

On Python 3, `pickle.loads` is able to take anything that conforms to
the (new) buffer protocol. So there has been no issue giving it an
`ndarray` to work with. Unfortunately, on Python 2, `pickle.loads`
requires a `bytes` object specifically and is not able to take any type
implementing the (new) buffer protocol. So we have been going ahead and
coercing everything to `bytes` on Python 2.

However it turns out that `cStringIO`'s `StringIO` on Python 2 does
support the buffer protocol. Thus a `StringIO` object can be created on
Python 2 without copying the data. While this still cannot be used with
`pickle.loads`, it can be used with `pickle.load`, which special cases
reading from `StringIO` leveraging the read function, which amounts to
sharing a pointer between `StringIO` and `pickle.loads`. Thus achieving
a no copying unpickler for Python 2.

ref: http://www.hydrogen18.com/blog/unpickling-buffers.html

ref: https://github.com/python/cpython/blob/2.7/Modules/cStringIO.c#L716
ref: https://github.com/python/cpython/blob/2.7/Modules/cStringIO.c#L681
ref: https://github.com/python/cpython/blob/2.7/Modules/cPickle.c#L614
ref: https://github.com/python/cpython/blob/2.7/Modules/cStringIO.c#L160

* Link this PR to `Pickle.decode` release note

Author: jakirkham

docs/release.rst

@@ -7,7 +7,7 @@ Release notes
 -----
 
 * Handle (new) buffer protocol conforming types in ``Pickle.decode``.
-  By :user:`John Kirkham <jakirkham>`, :issue:`143`.
+  By :user:`John Kirkham <jakirkham>`, :issue:`143`, :issue:`150`.
 
 * Fix other ``VLen*`` encode() methods to return numpy arrays as well.
   By :user:`John Kirkham <jakirkham>`, :issue:`144`.

numcodecs/pickles.py

@@ -6,11 +6,12 @@
 
 
 from .abc import Codec
-from .compat import PY2, ensure_bytes, ensure_contiguous_ndarray
+from .compat import PY2, ensure_contiguous_ndarray
 
 
 if PY2:  # pragma: py3 no cover
     import cPickle as pickle
+    from cStringIO import StringIO
 else:  # pragma: py2 no cover
     import pickle
 
@@ -48,12 +49,13 @@ def encode(self, buf):
         return pickle.dumps(buf, protocol=self.protocol)
 
     def decode(self, buf, out=None):
+        buf = ensure_contiguous_ndarray(buf)
+
         if PY2:  # pragma: py3 no cover
-            buf = ensure_bytes(buf)
+            dec = pickle.load(StringIO(buf))
         else:  # pragma: py2 no cover
-            buf = ensure_contiguous_ndarray(buf)
+            dec = pickle.loads(buf)
 
-        dec = pickle.loads(buf)
         if out is not None:
             np.copyto(out, dec)
             return out