Summary:
Adds a new crate `cpython-result`, which provides a `ResultExt` trait, which
extends the failure `Result` type to allow coversion to `PyResult` by
converting the error to an appropriate Python Exception.
Reviewed By: quark-zju
Differential Revision: D12980782
fbshipit-source-id: 44a63d31f9ecf2f77efa3b37c68f9a99eaf6d6fa