Add folly::Expected cython wrapper
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

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly