move ReadFile, WriteFile, and ReadDir into a separate io/ioutil package.

this breaks the dependency of package io on package bytes.

R=rsc
CC=golang-dev
https://golang.org/cl/163085

src/pkg/path/path.go

@@ -7,7 +7,7 @@
 package path
 
 import (
-	"io";
+	"io/ioutil";
 	"os";
 	"strings";
 )
@@ -155,7 +155,7 @@ func walk(path string, d *os.Dir, v Visitor, errors chan<- os.Error) {
 		return	// skip directory entries
 	}
 
-	list, err := io.ReadDir(path);
+	list, err := ioutil.ReadDir(path);
 	if err != nil {
 		if errors != nil {
 			errors <- err