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/single_test.go

@@ -14,6 +14,7 @@ import (
 	"github.com/davecgh/go-spew/spew"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/keychain"
+	"github.com/lightningnetwork/lnd/lnencrypt"
 	"github.com/lightningnetwork/lnd/lnwire"
 	"github.com/lightningnetwork/lnd/shachain"
 	"github.com/stretchr/testify/require"
@@ -207,7 +208,7 @@ func TestSinglePackUnpack(t *testing.T) {
 
 	singleChanBackup := NewSingle(channel, []net.Addr{addr1, addr2})
 
-	keyRing := &mockKeyRing{}
+	keyRing := &lnencrypt.MockKeyRing{}
 
 	versionTestCases := []struct {
 		// version is the pack/unpack version that we should use to
@@ -312,7 +313,7 @@ func TestSinglePackUnpack(t *testing.T) {
 func TestPackedSinglesUnpack(t *testing.T) {
 	t.Parallel()
 
-	keyRing := &mockKeyRing{}
+	keyRing := &lnencrypt.MockKeyRing{}
 
 	// To start, we'll create 10 new singles, and them assemble their
 	// packed forms into a slice.
@@ -361,7 +362,7 @@ func TestPackedSinglesUnpack(t *testing.T) {
 func TestSinglePackStaticChanBackups(t *testing.T) {
 	t.Parallel()
 
-	keyRing := &mockKeyRing{}
+	keyRing := &lnencrypt.MockKeyRing{}
 
 	// First, we'll create a set of random single, and along the way,
 	// create a map that will let us look up each single by its chan point.
@@ -407,8 +408,9 @@ func TestSinglePackStaticChanBackups(t *testing.T) {
 
 	// If we attempt to pack again, but force the key ring to fail, then
 	// the entire method should fail.
+	keyRing.Fail = true
 	_, err = PackStaticChanBackups(
-		unpackedSingles, &mockKeyRing{true},
+		unpackedSingles, &lnencrypt.MockKeyRing{Fail: true},
 	)
 	if err == nil {
 		t.Fatalf("pack attempt should fail")
@@ -432,7 +434,7 @@ func TestSingleUnconfirmedChannel(t *testing.T) {
 	channel.FundingBroadcastHeight = fundingBroadcastHeight
 
 	singleChanBackup := NewSingle(channel, []net.Addr{addr1, addr2})
-	keyRing := &mockKeyRing{}
+	keyRing := &lnencrypt.MockKeyRing{}
 
 	// Pack it and then unpack it again to make sure everything is written
 	// correctly, then check that the block height of the unpacked