bytes, strings: use rune

Various rune-based APIs change.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5306044
diff --git a/src/pkg/strings/replace_test.go b/src/pkg/strings/replace_test.go
index e337856..23c7e2e 100644
--- a/src/pkg/strings/replace_test.go
+++ b/src/pkg/strings/replace_test.go
@@ -159,12 +159,12 @@
 // BenchmarkByteByteMap compares byteByteImpl against Map.
 func BenchmarkByteByteMap(b *testing.B) {
 	str := Repeat("a", 100) + Repeat("b", 100)
-	fn := func(r int) int {
+	fn := func(r rune) rune {
 		switch r {
 		case 'a':
-			return int('A')
+			return 'A'
 		case 'b':
-			return int('B')
+			return 'B'
 		}
 		return r
 	}