Reformulate wrapping in terms of boxes, glue, and penalties

This is a complete rewrite of the core word wrapping functionality.
Before, we would step though the input string and (attempt to) keep
track of all aspects of the state. This didn't always work (see at
least #122, #158, #158, and #193) and it's inflexible.

This commit replaces the old algorithm with a new one which works on a
more abstract level. We now first

1. First split the input string into "words". A word is a substring of
   the original string, including any trailing whitespace.

2. We split each word according to the `WordSplitter`.

3. We then simply put the words into lines based on the display width.

This is slower than the previous algorithm. The `fill/1600` benchmark
shows that is now takes ~18 microseconds to wrap a 1600 character long
string. That is around 8 microseconds longer than before.

examples/interactive.rs

@@ -101,7 +101,7 @@ mod unix_only {
         )?;
         row += 2;
 
-        let mut lines = wrap(text, options).collect::<Vec<_>>();
+        let mut lines = wrap(text, options);
         if let Some(line) = lines.last() {
             // If `text` ends with a newline, the final wrapped line
             // contains this newline. This will in turn leave the

examples/layout.rs

@@ -17,7 +17,7 @@ fn main() {
 
     for width in 15..60 {
         options.width = width;
-        let lines = wrap(example, &options).collect::<Vec<_>>();
+        let lines = wrap(example, &options);
         if lines != prev_lines {
             let title = format!(" Width: {} ", width);
             println!(".{:-^1$}.", title, width + 2);