Remove support for chained key bindings

The editor currently supports emacs-style chained key binding with up to 3 keys (e.g. bind 'M-c M-b M-x' ...). This has always seemed like a misfeature to me. It also has some weird edge cases and requires a bunch of extra code.

It seems to me like this feature in emacs stems from the notion that every single editor feature should have a default key binding, but I much prefer to have bindings only for commonly used things and use command mode for everything else.

I'm putting this issue up in case anyone uses chained bindings and wants to argue the counterpoints, but if not, I'm leaning towards removing support for it entirely.