add bytes.IndexByte; common case we can make fast later.

also pick off the special case in strings.Index.   don't want strings.IndexByte
because the call site will very rarely need to allocate and we can handle the
test in the code itself.   bytes.IndexByte can avoid a common allocation.

R=rsc
CC=golang-dev
https://golang.org/cl/156091

src/pkg/bytes/bytes.go

@@ -98,6 +98,16 @@ func Index(s, sep []byte) 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 []byte, c byte) int {
+	for i, b := range s {
+		if b == 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 []byte) int {
 	n := len(sep);