io: add ByteWriter interface

API change.

R=golang-dev, dsymonds, nigeltao, rsc, r
CC=golang-dev
https://golang.org/cl/6760045

src/pkg/io/io.go

@@ -216,6 +216,11 @@ type ByteScanner interface {
 	UnreadByte() error
 }
 
+// ByteWriter is the interface that wraps the WriteByte method.
+type ByteWriter interface {
+	WriteByte(c byte) error
+}
+
 // RuneReader is the interface that wraps the ReadRune method.
 //
 // ReadRune reads a single UTF-8 encoded Unicode character