lnencrypt: Moves the crypto functions in the chanbackup package into its own package called lnencrypt

The functions inside of the crypto.go file in chanbackup (like EncryptPayloadToWriter and DecryptPayloadFromReader) can be used by a lot of things outside of just the chanbackup package. We can't just reference them directly from the chanbackup package because it's likely that it would generate circular dependencies. Therefore we need to move these functions into their own package to be referenced by chanbackup and whatever new functionality that needs them

chanbackup/multi_test.go

@@ -5,6 +5,7 @@ import (
 	"net"
 	"testing"
 
+	"github.com/lightningnetwork/lnd/lnencrypt"
 	"github.com/stretchr/testify/require"
 )
 
@@ -27,7 +28,7 @@ func TestMultiPackUnpack(t *testing.T) {
 		multi.StaticBackups = append(multi.StaticBackups, single)
 	}
 
-	keyRing := &mockKeyRing{}
+	keyRing := &lnencrypt.MockKeyRing{}
 
 	versionTestCases := []struct {
 		// version is the pack/unpack version that we should use to
@@ -93,14 +94,17 @@ func TestMultiPackUnpack(t *testing.T) {
 				)
 			}
 
+			encrypter, err := lnencrypt.KeyRingEncrypter(keyRing)
+			require.NoError(t, err)
+
 			// Next, we'll make a fake packed multi, it'll have an
 			// unknown version relative to what's implemented atm.
 			var fakePackedMulti bytes.Buffer
 			fakeRawMulti := bytes.NewBuffer(
 				bytes.Repeat([]byte{99}, 20),
 			)
-			err := encryptPayloadToWriter(
-				*fakeRawMulti, &fakePackedMulti, keyRing,
+			err = encrypter.EncryptPayloadToWriter(
+				fakeRawMulti.Bytes(), &fakePackedMulti,
 			)
 			if err != nil {
 				t.Fatalf("unable to pack fake multi; %v", err)
@@ -124,7 +128,7 @@ func TestMultiPackUnpack(t *testing.T) {
 func TestPackedMultiUnpack(t *testing.T) {
 	t.Parallel()
 
-	keyRing := &mockKeyRing{}
+	keyRing := &lnencrypt.MockKeyRing{}
 
 	// First, we'll make a new unpacked multi with a random channel.
 	testChannel, err := genRandomOpenChannelShell()