crypto/sha{256,512}: unname result parameters for consistency

Sum224 and Sum256 didn't look the same at:

    https://golang.org/pkg/crypto/sha256/

Now they match. Likewise with sha512's funcs.

Per:
https://github.com/golang/go/wiki/CodeReviewComments#named-result-parameters

Change-Id: I6b88c8ef15141c78a6cddeb0960b3ad52db34244
Reviewed-on: https://go-review.googlesource.com/c/go/+/322329
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Trust: Katie Hockman <katie@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/crypto/sha256/sha256.go

@@ -259,12 +259,12 @@ func Sum256(data []byte) [Size]byte {
 }
 
 // Sum224 returns the SHA224 checksum of the data.
-func Sum224(data []byte) (sum224 [Size224]byte) {
+func Sum224(data []byte) [Size224]byte {
 	var d digest
 	d.is224 = true
 	d.Reset()
 	d.Write(data)
 	sum := d.checkSum()
-	copy(sum224[:], sum[:Size224])
-	return
+	ap := (*[Size224]byte)(sum[:])
+	return *ap
 }

src/crypto/sha512/sha512.go

@@ -337,31 +337,31 @@ func Sum512(data []byte) [Size]byte {
 }
 
 // Sum384 returns the SHA384 checksum of the data.
-func Sum384(data []byte) (sum384 [Size384]byte) {
+func Sum384(data []byte) [Size384]byte {
 	d := digest{function: crypto.SHA384}
 	d.Reset()
 	d.Write(data)
 	sum := d.checkSum()
-	copy(sum384[:], sum[:Size384])
-	return
+	ap := (*[Size384]byte)(sum[:])
+	return *ap
 }
 
 // Sum512_224 returns the Sum512/224 checksum of the data.
-func Sum512_224(data []byte) (sum224 [Size224]byte) {
+func Sum512_224(data []byte) [Size224]byte {
 	d := digest{function: crypto.SHA512_224}
 	d.Reset()
 	d.Write(data)
 	sum := d.checkSum()
-	copy(sum224[:], sum[:Size224])
-	return
+	ap := (*[Size224]byte)(sum[:])
+	return *ap
 }
 
 // Sum512_256 returns the Sum512/256 checksum of the data.
-func Sum512_256(data []byte) (sum256 [Size256]byte) {
+func Sum512_256(data []byte) [Size256]byte {
 	d := digest{function: crypto.SHA512_256}
 	d.Reset()
 	d.Write(data)
 	sum := d.checkSum()
-	copy(sum256[:], sum[:Size256])
-	return
+	ap := (*[Size256]byte)(sum[:])
+	return *ap
 }