Initial design of Terminal class

This greatly overhauls the output path and does some tweaks to the input path.

The main point of this is to move terminal-specific stuff into a subclass of the new Terminal class. This new terminal class monitors all output and maintains the current state. It also provides the API used by Synchronet to control the remote terminal.

On the input side, only the parsing of received sequences is moved into the Terminal class.

Importantly charset translation and conversion is not part of the Terminal class. When that needs to be refactored, it should be a different class doing different things in difference places.