Add folly::Function cython wrapper

Summary:
Add basic wrapper to support wrapping C++ APIs that contain
folly::Function in their interfaces. Does not support calling operator() from
within Cython, but only pass through of folly::Function<> to such APIs.

Reviewed By: nanshu

Differential Revision: D18526112

fbshipit-source-id: d219d480e59464099a109e5ef30d015151588b1f

folly/python/function.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/Function.h" namespace "folly" nogil:
+    cdef cppclass cFunction "folly::Function"[T]:
+        cFunction()
+        cFunction(cFunction&&)
+        
+        bint operator bool()