Let invoice.Encode receive a function which hashes itself the message

Modify the SignCompact function passed to invoice.Encode to receive the
message before it's hashed and hash it itself.
With this modification, the SignMessage rpc function from the signrpc
subserver can be used and an invoice can be encoded outside of lnd.

zpay32/encode.go

@@ -70,12 +70,11 @@ func (invoice *Invoice) Encode(signer MessageSigner) (string, error) {
 	}
 
 	toSign := append([]byte(hrp), taggedFieldsBytes...)
-	hash := chainhash.HashB(toSign)
 
 	// We use compact signature format, and also encoded the recovery ID
 	// such that a reader of the invoice can recover our pubkey from the
 	// signature.
-	sign, err := signer.SignCompact(hash)
+	sign, err := signer.SignCompact(toSign)
 	if err != nil {
 		return "", err
 	}
@@ -95,6 +94,7 @@ func (invoice *Invoice) Encode(signer MessageSigner) (string, error) {
 				"signature: %v", err)
 		}
 
+		hash := chainhash.HashB(toSign)
 		valid := signature.Verify(hash, invoice.Destination)
 		if !valid {
 			return "", fmt.Errorf("signature does not match " +