Vi-like editing with the Xi core, written in pure Rust

ViXi is written with the idea that Vi's mode switching and keystroke remapping features can be managed in a separate library and dropped into either the Xi core library or into any Xi frontend.

The strict separation of concerns implicit in this design is intended to avoid a lot of the difficulties found in extending and maintaining the Vim and Neovim codebases.

By convention, generic type names follow this pattern:

• K is a keystroke, implementing Copy and Ord.
• T is an arbitrary type, typically stored as a value in a map.
• Op is an arbitrary operation type (enums from op.rs).

• Decide on an interface between ViXi and a ViXi object's owner.
• Implement MVP operations for Normal, Insert, and Op-pending.