lnwire: add new SerializedSize method to all wire messages

This'll be useful for the bandwidth based rate limiting we'll implement
in the next commit.

lnwire/custom.go

@@ -73,6 +73,10 @@ type Custom struct {
 // interface.
 var _ Message = (*Custom)(nil)
 
+// A compile time check to ensure Custom implements the lnwire.SizeableMessage
+// interface.
+var _ SizeableMessage = (*Custom)(nil)
+
 // NewCustom instantiates a new custom message.
 func NewCustom(msgType MessageType, data []byte) (*Custom, error) {
 	if msgType < CustomTypeStart && !IsCustomOverride(msgType) {
@@ -117,3 +121,10 @@ func (c *Custom) Decode(r io.Reader, pver uint32) error {
 func (c *Custom) MsgType() MessageType {
 	return c.Type
 }
+
+// SerializedSize returns the serialized size of the message in bytes.
+//
+// This is part of the lnwire.SizeableMessage interface.
+func (c *Custom) SerializedSize() (uint32, error) {
+	return MessageSerializedSize(c)
+}