Move PPExt functions from IO to CoreM

[Kha]

We currently lose all trace messages when going through the PPExt indirection layer. While the indirection still makes sense even in the new frontend to avoid a dependency cycle, we should change the base monad after removing the old frontend.

[Kha]

I'm closing this since this will simply lead to another dependency cycle: PPExt -> CoreM -> Message -> PPExt. We would have to split at least Message and Util.Trace to circumvent it. It's probably easier to only use direct invocations when testing and tracing the pretty printer.