`crossterm`, `termion`, and `termwiz` can now be accessed as
`ratatui::{crossterm, termion, termwiz}` respectively. This makes it
possible to just add the Ratatui crate as a dependency and use the
backend of choice without having to add the backend crates as
dependencies.
To update existing code, replace all instances of `crossterm::` with
`ratatui::crossterm::`, `termion::` with `ratatui::termion::`, and
`termwiz::` with `ratatui::termwiz::`.
