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/commit_sig.go

@@ -71,12 +71,19 @@ func (c *CommitSig) Decode(r io.Reader, pver uint32) error {
 //
 // This is part of the lnwire.Message interface.
 func (c *CommitSig) Encode(w *bytes.Buffer, pver uint32) error {
-	return WriteElements(w,
-		c.ChanID,
-		c.CommitSig,
-		c.HtlcSigs,
-		c.ExtraData,
-	)
+	if err := WriteChannelID(w, c.ChanID); err != nil {
+		return err
+	}
+
+	if err := WriteSig(w, c.CommitSig); err != nil {
+		return err
+	}
+
+	if err := WriteSigs(w, c.HtlcSigs); err != nil {
+		return err
+	}
+
+	return WriteBytes(w, c.ExtraData)
 }
 
 // MsgType returns the integer uniquely identifying this message type on the