Auto merge of #18355 - servo:no-fmt, r=emilio

Reduce usage of fmt in serialization and error reporting

`format!` and `write!` create a somewhat-heavyweight `Formatting` struct and use dynamic dispatch to call into impls of `Dispaly` and related traits. The former also allocates an intermediate string that is sometimes unnecessary.

I started looking into this from https://bugzilla.mozilla.org/show_bug.cgi?id=1355599, but I expect the impact there will be small to insignificant. It might be a slightly less so on parsing (error reporting).

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components/style/values/specified/length.rs

@@ -424,7 +424,11 @@ impl ToCss for NoCalcLength {
             NoCalcLength::FontRelative(length) => length.to_css(dest),
             NoCalcLength::ViewportPercentage(length) => length.to_css(dest),
             /* This should only be reached from style dumping code */
-            NoCalcLength::ServoCharacterWidth(CharacterWidth(i)) => write!(dest, "CharWidth({})", i),
+            NoCalcLength::ServoCharacterWidth(CharacterWidth(i)) => {
+                dest.write_str("CharWidth(")?;
+                i.to_css(dest)?;
+                dest.write_char(')')
+            }
             #[cfg(feature = "gecko")]
             NoCalcLength::Physical(length) => length.to_css(dest),
         }