Introduce mode rings

[countvajhula]

Summary of Changes

Instead of towers containing modes, we'd like them to contain mode rings. This is a natural and intuitive structure to use for modes and can make things pretty nice.

This uses the dynaring package.

The goals of this PR are:

• Currently, tower "levels" contain modes. Change this so that they contain mode rings (which could have a single element).
• Define a default UX in terms of keybinding flows, maybe something like what Tommy uses.
• Define a visual (and textual) representation of a mode ring which can be appropriately edited in meta modes.
• Review all instances of "escaping" and "entering" which assume a tower topology, and see if they should use ring rotation instead.
• Review and update the default editing structures to make appropriate use of rings (the towers including lisp tower, the complexes, as well as the structures used in meta modes).
• Review interop in Symex to see if those need to be changed (e.g. to use rings instead of escaping higher).
• Document the various structures that may be relevant for users, and how to define keybindings for them to support whatever UX/topology they want.
• Document the required .emacs.d config updates.

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