Generalize from IO to MonadUnliftIO m => m

[Fizzixnerd]

This generalizes the monad that Segments run in from IO to MonadUnliftIO m => m. This has many benefits when interacting with the conduit library. For example, it was not possible to use resourcet to handle file resources when outputting to files. With this change, the following code becomes valid:

runResourceT $ run $ do
  ls $| (conduit $ sinkFile "ls.txt")

This is opposed to requiring before to manage the file Handle yourself manually and passing it down, risking closing it too early.

Since redirecting to files is a large part of writing shell scripts, I believe it is worth the cost of 1) breaking the API and 2) slightly increased complexity to integrate this change.

[psibi]

Sorry, that it took quite a bit of time to review this. The change LGTM apart from one comment.