strings: remove allocations in Split(s, "")

BenchmarkSplit1     77984460     24131380  -69.06%

R=golang-dev, rsc, minux.ma, dave, extemporalgenome
CC=golang-dev
https://golang.org/cl/7458043

src/pkg/strings/strings.go

@@ -26,7 +26,11 @@ func explode(s string, n int) []string {
 	i, cur := 0, 0
 	for ; i+1 < n; i++ {
 		ch, size = utf8.DecodeRuneInString(s[cur:])
-		a[i] = string(ch)
+		if ch == utf8.RuneError {
+			a[i] = string(utf8.RuneError)
+		} else {
+			a[i] = s[cur : cur+size]
+		}
 		cur += size
 	}
 	// add the rest, if there is any