Export one more function

Added :get-key-structs to the export list.
Converted curses interface to new API

Finished moving the ncurses UI to the new API.
Added function to handle UI interaction

Now that we've written an API involving a UI base class with
implementations derived from it, there is a new function that works
through that API, calling the appropriate generic functions to talk to
the user through the selected UI.
Added a function

Needed a new function that returns not just the abbreviations, but the
entire key struct list.
Added more exports

Exported several functions related to the UI interface.