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/piecewise_linear.rs

@@ -39,7 +39,7 @@ impl ToCss for PiecewiseLinearFunctionEntry {
         W: fmt::Write,
     {
         self.y.to_css(dest)?;
-        dest.write_str(" ")?;
+        dest.write_char(' ')?;
         Percentage(self.x).to_css(dest)
     }
 }