Add folly::Expected cython wrapper

Summary:
Add new cython wrapper class to expose folly::Expected to other
cython_libraries

Reviewed By: jkedgar

Differential Revision: D18523642

fbshipit-source-id: 0dfb4820b2613361b7c1af05b89b5231a1bfecbc

folly/python/expected.pxd

+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+cdef extern from "folly/Expected.h" namespace "folly" nogil:
+    cdef cppclass cExpected "folly::Expected"[T, E]:
+        cExpected(T&& val)
+        bint hasValue() const
+        bint hasError() const
+        T value()
+        E error()
+        cExpected operator bool() const