slog: eliminate needsQuotingSet

Delete the set of bytes that need quoting in TextHandler, because it
is almost identical to the set for JSON. Use JSONHandler's safeSet
with a few exceptions.

Updates golang#56345.

Change-Id: Iff6d309c067affef2e5ecfcebd6e1bb8f00f95b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/478198
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/log/slog/text_handler.go

@@ -137,7 +137,9 @@ func needsQuoting(s string) bool {
 	for i := 0; i < len(s); {
 		b := s[i]
 		if b < utf8.RuneSelf {
-			if needsQuotingSet[b] {
+			// Quote anything except a backslash that would need quoting in a
+			// JSON string, as well as space and '='
+			if b != '\\' && (b == ' ' || b == '=' || !safeSet[b]) {
 				return true
 			}
 			i++
@@ -151,17 +153,3 @@ func needsQuoting(s string) bool {
 	}
 	return false
 }
-
-var needsQuotingSet = [utf8.RuneSelf]bool{
-	'"': true,
-	'=': true,
-}
-
-func init() {
-	for i := 0; i < utf8.RuneSelf; i++ {
-		r := rune(i)
-		if unicode.IsSpace(r) || !unicode.IsPrint(r) {
-			needsQuotingSet[i] = true
-		}
-	}
-}