[pfff] Support type annotations in spatch

Summary:
To change `Continuation<X>` into `Generator<int, X, void>`.
Only nontrivial part is to allow `X` to match things like `Vector<int>`.

Test Plan: Test cases included.

Reviewers: pad

Reviewed By: pad

Subscribers: eletuchy

Differential Revision: https://phabricator.fb.com/D1452550

Tasks: 4534682