We weren't quite right with handling the cursor around double-width
characters. We're now a bit more robust at this because we're
clustering and taking pains to consider the printable width of the
cell as well as the width of the shaped (eg: with contextual ligatures)
glyph.
There may still be issues with contextual ligatures of length 3,
but I haven't managed to nail down exactly the issue yet.
This eliminates the zsh reversed % artifact issue.
Added a feature flag to turn on diagnostics even in release mode;
`cargo run --release --features debug-escape-sequences`
