When trying to use the custom handle infrastructure, hGetContents fails like so:

*** Exception: ToDo: hGetBuf

This exception occurs twice in GHC.IO.Handle.Text

The handle implementation I'm using is attached.

It would be neat if I could pass along some witness that my device implements RawDevice, then we could just run the same code that we use for file-descriptors. But I'd be happy enough with a general solution, as I just plan to use this for testing.