Add folly::Function cython wrapper
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

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