import folly::down_cast to Python/Cython

Summary: In Cython, use `from folly.cast cimport down_cast` to import the function.

Reviewed By: yfeldblum

Differential Revision: D17652305

fbshipit-source-id: 6b038c5ac6c2a8ecd8e95f33ae3a37647a2bfdff

folly/python/lang/cast.pxd

+# Copyright 2019-present Facebook, Inc.
+#
+# 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/lang/Cast.h" namespace "folly" nogil:
+    cdef T down_cast_ref "folly::down_cast"[T, S](S ref)
+    cdef T* down_cast_ptr "folly::down_cast"[T, S](S* ptr)