Move `schedule()` from `scheduler` into `task` (#858)

* Allows the `task` crate itself to directly invoke `schedule()`,
  which is important for upcoming efficiency improvements to
  regular and async task management (e.g., waking a task that was
  blocked on another task for the purpose of `join`).
  * Also allows future PRs to avoid related cyclic dependencies.

* The `scheduler` crate still provides the same scheduling functionality
  by re-exporting the same function from `task`, in order to avoid
  making changes to crates that depend on `schedule()`. 7fc1719d8d8ffb161397a08608a9eed0ce1b5443