This patch does two improvements:

- We now show ToDos in `CoreDoPasses`. This is pretty important,
  otherwise `CoreDoPasses` makes debugging impossible in some cases.

- Before running ToDos we run a cleanup pass on ToDos to remove
  `CoreDoNothing`s and flatten `CoreDoPasses`. This removes a lot of
  noise from `[CoreToDo]` argument passed to plugins.

Reviewers: simonpj, bgamari, austin

Reviewed By: bgamari, austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1548