encoding/base{32,64}: add examples.

Fixes #4136.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/6615061

src/pkg/encoding/base32/example_test.go

+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package base32_test
+
+import (
+	"encoding/base32"
+	"fmt"
+)
+
+func ExampleEncoding_EncodeToString() {
+	data := []byte("any + old & data")
+	str := base32.StdEncoding.EncodeToString(data)
+	fmt.Println(str)
+	// Output:
+	// MFXHSIBLEBXWYZBAEYQGIYLUME======
+}
+
+func ExampleEncoding_DecodeString() {
+	str := "ONXW2ZJAMRQXIYJAO5UXI2BAAAQGC3TEEDX3XPY="
+	data, err := base32.StdEncoding.DecodeString(str)
+	if err != nil {
+		fmt.Println("error:", err)
+		return
+	}
+	fmt.Printf("%q\n", data)
+	// Output:
+	// "some data with \x00 and \ufeff"
+}

src/pkg/encoding/base64/example_test.go

+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package base64_test
+
+import (
+	"encoding/base64"
+	"fmt"
+)
+
+func ExampleEncoding_EncodeToString() {
+	data := []byte("any + old & data")
+	str := base64.StdEncoding.EncodeToString(data)
+	fmt.Println(str)
+	// Output:
+	// YW55ICsgb2xkICYgZGF0YQ==
+}
+
+func ExampleEncoding_DecodeString() {
+	str := "c29tZSBkYXRhIHdpdGggACBhbmQg77u/"
+	data, err := base64.StdEncoding.DecodeString(str)
+	if err != nil {
+		fmt.Println("error:", err)
+		return
+	}
+	fmt.Printf("%q\n", data)
+	// Output:
+	// "some data with \x00 and \ufeff"
+}