style: Use write_char in place of write_str when serializing single-character literals

Generated by running

  find servo/components/style -name "*.rs" -exec perl -p -i -e "s/write_str\(\"(.)\"\)/write_char('\1')/g" {} \;

(and then added `use std::fmt::Write;` in a couple of places to fix build errors that arose).

Differential Revision: https://phabricator.services.mozilla.com/D168217

components/style/values/computed/text.rs

@@ -161,7 +161,7 @@ impl ToCss for TextOverflow {
             self.second.to_css(dest)?;
         } else {
             self.first.to_css(dest)?;
-            dest.write_str(" ")?;
+            dest.write_char(' ')?;
             self.second.to_css(dest)?;
         }
         Ok(())