lnwire: refactor Encode to use specific writers - I

This commit takes 10 types of messages and refactors their Encode method
to use specific writers. The following commits will refactor the rest.

lnwire/announcement_signatures.go

@@ -66,13 +66,23 @@ func (a *AnnounceSignatures) Decode(r io.Reader, pver uint32) error {
 //
 // This is part of the lnwire.Message interface.
 func (a *AnnounceSignatures) Encode(w *bytes.Buffer, pver uint32) error {
-	return WriteElements(w,
-		a.ChannelID,
-		a.ShortChannelID,
-		a.NodeSignature,
-		a.BitcoinSignature,
-		a.ExtraOpaqueData,
-	)
+	if err := WriteChannelID(w, a.ChannelID); err != nil {
+		return err
+	}
+
+	if err := WriteShortChannelID(w, a.ShortChannelID); err != nil {
+		return err
+	}
+
+	if err := WriteSig(w, a.NodeSignature); err != nil {
+		return err
+	}
+
+	if err := WriteSig(w, a.BitcoinSignature); err != nil {
+		return err
+	}
+
+	return WriteBytes(w, a.ExtraOpaqueData)
 }
 
 // MsgType returns the integer uniquely identifying this message type on the