strings: add IndexByte, for consistency with bytes package

I always forget which package has it.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/12214044

src/pkg/strings/strings.go

@@ -130,14 +130,7 @@ func Index(s, sep string) int {
 	case n == 0:
 		return 0
 	case n == 1:
-		c := sep[0]
-		// special case worth making fast
-		for i := 0; i < len(s); i++ {
-			if s[i] == c {
-				return i
-			}
-		}
-		return -1
+		return IndexByte(s, sep[0])
 	case n == len(s):
 		if sep == s {
 			return 0
@@ -167,6 +160,16 @@ func Index(s, sep string) int {
 	return -1
 }
 
+// IndexByte returns the index of the first instance of c in s, or -1 if c is not present in s.
+func IndexByte(s string, c byte) int {
+	for i := 0; i < len(s); i++ {
+		if s[i] == c {
+			return i
+		}
+	}
+	return -1
+}
+
 // LastIndex returns the index of the last instance of sep in s, or -1 if sep is not present in s.
 func LastIndex(s, sep string) int {
 	n := len(sep)