Make it reloadable

Windows need their own top-of-window-mtype and point location

Preserve column on up/down better. This includes dealing with tab expansion

Bundle up all mutable command context into a single record, to let the
command handler have everything it needs even in the face of change

Need line wrap of some kind.

unknown-escape-sequence contents are byte strings, and keyseq parsing
and printing should reflect that.

something to do with framing is broken, causing odd scroll-lock
behaviour near the end of the demo file.

Catch and handle SIGWINCH.
See http://man7.org/tlpi/code/online/dist/tty/demo_SIGWINCH.c.html