TermBuf::Input::Reader::Inbound

What arrives on the inbound channel. A union rather than plain Bytes so that the end of input is a value like any other, and so that a wake-up that came from nowhere near the device has somewhere to go.

The reader itself only ever sends Bytes and one Eof. A Timers::Tick comes from a timer fibre and a Signals::Signalled from Crystal's signal fibre, and both arrive here rather than on channels of their own so that they are ordered against the bytes: what the terminal said before a timer was armed, or before a signal landed, is always dispatched before it.