Store Styled with contiguous string

The previous implementation of Styled used chunks that consisted of a
String and a ContentStyle. The current implementation instead stores a
single String and chunks consisting of a ContentStyle and an ending
index.

This implementation may reduce allocations and makes width-related
operations easier, for example getting the width of a Styled with its
whitespace trimmed.

examples/text_wrapping.rs

@@ -31,7 +31,7 @@ fn draw(f: &mut Frame) {
     );
 
     let breaks = f.wrap(text, f.size().width.into());
-    let lines = Styled::default().then(text).split_at_indices(&breaks);
+    let lines = Styled::new_plain(text).split_at_indices(&breaks);
     for (i, mut line) in lines.into_iter().enumerate() {
         line.trim_end();
         f.write(Pos::new(0, i as i32), line);