Refactor the python system_impl module.

mojo/public/python/mojo/c_thunks.pxd

Index: mojo/public/python/mojo/c_thunks.pxd
diff --git a/mojo/public/python/mojo/c_thunks.pxd b/mojo/public/python/mojo/c_thunks.pxd
new file mode 100644
index 0000000000000000000000000000000000000000..f2d83b0c5ed025cdf92f6c5a4bf59ca9fde9d330
--- /dev/null
+++ b/mojo/public/python/mojo/c_thunks.pxd
@@ -0,0 +1,17 @@
+# Copyright 2014 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# distutils: language = c++
+
+
+# If the definition below is not present, cython-compiled modules do not expose
+# an init method as they should.
+cdef extern from "third_party/cython/python_export.h":
+  pass
+
+cdef extern from "mojo/public/platform/native/system_thunks.h" nogil:
+  cdef struct MojoSystemThunks:
+    pass
+
+cdef extern size_t MojoSetSystemThunks(const MojoSystemThunks* system_thunks)