make lots of functions const that weren't before; it is also technica…

…lly possible to make the style difference calculations const (using while loops, but for now I have not pursued that

examples/hyperlink.rs

@@ -10,7 +10,7 @@ fn main() {
     let link = Color::Blue
         .underline()
         .paint("Link to example.com")
-        .hyperlink("https://example.com");
+        .hyperlink_content("https://example.com");
 
     println!("{}", link);
     sleep(sleep_ms);

src/difference.rs

@@ -157,7 +157,7 @@ impl Style {
         } else {
             // No colors or formatting options were turned off. But there might
             // have been things turned on, or colors that have changed. This
-            // case handles that.
+            // case
             let turned_off_in_next = BoolStyle::turned_off(self.into(), next.into());
             if turned_off_in_next.formats.is_empty() && turned_off_in_next.coloring.is_empty() {
                 let turned_on_from_self = BoolStyle::turned_on(self.into(), next.into());