crypto/internal/boring: replace slice growth loop with slices.Grow

If dst is nil and len(plaintext) is 10MB, the slice growth loop
allocates about 70MB. slices.Grow only allocates 10MB.
Noticed on a real profile in a program built with boringcrypto.

Change-Id: Iecc727b961273d4f2d553669d43014c507e25df8
Reviewed-on: https://go-review.googlesource.com/c/go/+/744400
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>

src/crypto/internal/boring/aes.go

@@ -48,6 +48,7 @@ import (
 	"crypto/cipher"
 	"errors"
 	"runtime"
+	"slices"
 	"strconv"
 	"unsafe"
 )
@@ -323,9 +324,7 @@ func (g *aesGCM) Seal(dst, nonce, plaintext, additionalData []byte) []byte {
 
 	// Make room in dst to append plaintext+overhead.
 	n := len(dst)
-	for cap(dst) < n+len(plaintext)+gcmTagSize {
-		dst = append(dst[:cap(dst)], 0)
-	}
+	dst = slices.Grow(dst, len(plaintext)+gcmTagSize)
 	dst = dst[:n+len(plaintext)+gcmTagSize]
 
 	// Check delayed until now to make sure len(dst) is accurate.