time: optimize quote using byte(c) for ASCII

Since c < runeSelf && c >= ' ' (i.e., 32 <= c < 128), using buf = append(buf, byte(c)) instead of buf = append(buf, string(c)...) is a better choice, as it provides better performance.

Change-Id: Ic0ab25c71634a1814267f4d85be2ebd8a3d44676
GitHub-Last-Rev: 5445b54771
GitHub-Pull-Request: golang/go#72820
Reviewed-on: https://go-review.googlesource.com/c/go/+/657055
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

src/time/format.go

@@ -891,7 +891,7 @@ func quote(s string) string {
 			if c == '"' || c == '\\' {
 				buf = append(buf, '\\')
 			}
-			buf = append(buf, string(c)...)
+			buf = append(buf, byte(c))
 		}
 	}
 	buf = append(buf, '"')