crypto/md5, crypto/sha1, crypto/sha256: add examples for checksumming a file

Updates #16360.

Change-Id: I75714d2b5f095fe39fd81edfa6dd9e44d7c44da1
Reviewed-on: https://go-review.googlesource.com/29375
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/md5/example_test.go

@@ -8,6 +8,8 @@ import (
 	"crypto/md5"
 	"fmt"
 	"io"
+	"log"
+	"os"
 )
 
 func ExampleNew() {
@@ -23,3 +25,18 @@ func ExampleSum() {
 	fmt.Printf("%x", md5.Sum(data))
 	// Output: b0804ec967f48520697662a204f5fe72
 }
+
+func ExampleNew_file() {
+	f, err := os.Open("file.txt")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer f.Close()
+
+	h := md5.New()
+	if _, err := io.Copy(h, f); err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("%x", h.Sum(nil))
+}

src/crypto/sha1/example_test.go

@@ -8,6 +8,8 @@ import (
 	"crypto/sha1"
 	"fmt"
 	"io"
+	"log"
+	"os"
 )
 
 func ExampleNew() {
@@ -23,3 +25,18 @@ func ExampleSum() {
 	fmt.Printf("% x", sha1.Sum(data))
 	// Output: af 06 49 23 bb f2 30 15 96 aa c4 c2 73 ba 32 17 8e bc 4a 96
 }
+
+func ExampleNew_file() {
+	f, err := os.Open("file.txt")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer f.Close()
+
+	h := sha1.New()
+	if _, err := io.Copy(h, f); err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("% x", h.Sum(nil))
+}

src/crypto/sha256/example_test.go

@@ -7,6 +7,9 @@ package sha256_test
 import (
 	"crypto/sha256"
 	"fmt"
+	"io"
+	"log"
+	"os"
 )
 
 func ExampleSum256() {
@@ -21,3 +24,18 @@ func ExampleNew() {
 	fmt.Printf("%x", h.Sum(nil))
 	// Output: a948904f2f0f479b8f8197694b30184b0d2ed1c1cd2a1ec0fb85d299a192a447
 }
+
+func ExampleNew_file() {
+	f, err := os.Open("file.txt")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer f.Close()
+
+	h := sha256.New()
+	if _, err := io.Copy(h, f); err != nil {
+		log.Fatal(err)
+	}
+
+	fmt.Printf("%x", h.Sum(nil))
+}