multi: refactor SignDigestCompact into SignMessageCompact

To make it possible to use a remote lnrpc server as a signer for our
wallet, we need to change our main interface to sign the message instead
of the message's digest. Otherwise we'd need to alter the
lnrpc.SignMessage RPC to accept a digest instead of only the message
which has security implications.

fuzz/zpay32/encode.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/btcsuite/btcd/btcec"
 	"github.com/btcsuite/btcd/chaincfg"
+	"github.com/btcsuite/btcd/chaincfg/chainhash"
 	"github.com/lightningnetwork/lnd/zpay32"
 )
 
@@ -31,9 +32,11 @@ func Fuzz_encode(data []byte) int {
 	// Then, initialize the testMessageSigner so we can encode out
 	// invoices with this private key.
 	testMessageSigner := zpay32.MessageSigner{
-		SignCompact: func(hash []byte) ([]byte, error) {
-			sig, err := btcec.SignCompact(btcec.S256(),
-				testPrivKey, hash, true)
+		SignCompact: func(msg []byte) ([]byte, error) {
+			hash := chainhash.HashB(msg)
+			sig, err := btcec.SignCompact(
+				btcec.S256(), testPrivKey, hash, true,
+			)
 			if err != nil {
 				return nil, fmt.Errorf("can't sign the "+
 					"message: %v", err)