Write char::DebugEscape sequences using write_str

Instead of writing each `char` of an escape sequence one by one,
this delegates to `Display`, which uses `write_str` internally
in order to write the whole escape sequence at once.

library/core/benches/str/debug.rs

@@ -44,7 +44,7 @@ fn ascii_escapes(b: &mut Bencher) {
     assert_fmt(
         s,
         r#""some\tmore\tascii\ttext\nthis time with some \"escapes\", also 64 byte""#,
-        21,
+        15,
     );
     b.iter(|| {
         black_box(format!("{:?}", black_box(s)));
@@ -72,7 +72,7 @@ fn mostly_unicode(b: &mut Bencher) {
 #[bench]
 fn mixed(b: &mut Bencher) {
     let s = "\"❤️\"\n\"hűha ez betű\"\n\"кириллических букв\".";
-    assert_fmt(s, r#""\"❤\u{fe0f}\"\n\"hűha ez betű\"\n\"кириллических букв\".""#, 36);
+    assert_fmt(s, r#""\"❤\u{fe0f}\"\n\"hűha ez betű\"\n\"кириллических букв\".""#, 21);
     b.iter(|| {
         black_box(format!("{:?}", black_box(s)));
     });

library/core/src/fmt/mod.rs

@@ -2409,9 +2409,7 @@ impl Debug for str {
             // If char needs escaping, flush backlog so far and write, else skip
             if esc.len() != 1 {
                 f.write_str(&self[from..i])?;
-                for c in esc {
-                    f.write_char(c)?;
-                }
+                Display::fmt(&esc, f)?;
                 from = i + c.len_utf8();
             }
         }
@@ -2431,13 +2429,12 @@ impl Display for str {
 impl Debug for char {
     fn fmt(&self, f: &mut Formatter<'_>) -> Result {
         f.write_char('\'')?;
-        for c in self.escape_debug_ext(EscapeDebugExtArgs {
+        let esc = self.escape_debug_ext(EscapeDebugExtArgs {
             escape_grapheme_extended: true,
             escape_single_quote: true,
             escape_double_quote: false,
-        }) {
-            f.write_char(c)?
-        }
+        });
+        Display::fmt(&esc, f)?;
         f.write_char('\'')
     }
 }