Changed `send` and `seal` to be `IO` actions

The `STM` requirement for `send` actually makes most use cases more difficult.
For example, to convert an action of type `a -> IO Bool` to an `Output`, you
have to use `spawn'`, which is overkill.  There seems to be no benefit to
preserving `STM` and there are many benefits to simplifying it to `IO`.

I'm leaving this pull request open for a week for others to review and comment on.  If there are no comments then I will merge it into `master` and upload to Hackage as version 2.1.0.