Move optional python extensions from fbthrift to folly and add missing pyx files to thrift

Summary: This is another step of moving the python extensions for folly that are currently defined in thrift's codebase. Another step for getting us closer on unblocking LogDevice to use the python3 thrift client in open-source.

Reviewed By: yfeldblum, vitaut

Differential Revision: D16221352

fbshipit-source-id: bc24fce073d67397f20f28f79c68512444704402

folly/CMakeLists.txt

@@ -43,8 +43,20 @@ if (PYTHON_EXTENSIONS)
     "range.pxd"
     "iobuf.pxd"
     "iobuf.pyx"
+    "optional.pyx"
     "__init__.pxd"
     "AsyncioExecutor.h")
+  message(
+    STATUS
+    "Linking ${CMAKE_CURRENT_SOURCE_DIR}/python/lang/cast.pxd "
+    "to ${_cybld}/folly/cast.pxd"
+  )
+  add_custom_target("create_folly_link_cast.pxd" ALL
+    COMMAND
+      ${CMAKE_COMMAND} -E create_symlink
+      ${CMAKE_CURRENT_SOURCE_DIR}/python/lang/cast.pxd
+      "${_cybld}/folly/cast.pxd"
+  )
     message(
       STATUS
       "Linking ${CMAKE_CURRENT_SOURCE_DIR}/python/${_src} "