all: use io.ByteWriter now that it exists

R=golang-dev, minux.ma
CC=golang-dev
https://golang.org/cl/7079043

src/pkg/compress/lzw/writer.go

@@ -13,7 +13,7 @@ import (
 
 // A writer is a buffered, flushable writer.
 type writer interface {
-	WriteByte(byte) error
+	io.ByteWriter
 	Flush() error
 }
 

src/pkg/exp/html/render.go

@@ -14,7 +14,7 @@ import (
 
 type writer interface {
 	io.Writer
-	WriteByte(byte) error
+	io.ByteWriter
 	WriteString(string) (int, error)
 }
 

src/pkg/image/jpeg/writer.go

@@ -210,8 +210,8 @@ func init() {
 // writer is a buffered writer.
 type writer interface {
 	Flush() error
-	Write([]byte) (int, error)
-	WriteByte(byte) error
+	io.Writer
+	io.ByteWriter
 }
 
 // encoder encodes an image to the JPEG format.