lnwire: refactor Encode to use specific writers - III

This commit refactors the remaining usage of WriteElements. By
replacing the interface types with concrete types for the params used in
the methods, most of the encoding of the messages now takes zero heap
allocations.

lnwire/extra_bytes.go

@@ -18,7 +18,7 @@ type ExtraOpaqueData []byte
 // Encode attempts to encode the raw extra bytes into the passed io.Writer.
 func (e *ExtraOpaqueData) Encode(w *bytes.Buffer) error {
 	eBytes := []byte((*e)[:])
-	if err := WriteElements(w, eBytes); err != nil {
+	if err := WriteBytes(w, eBytes); err != nil {
 		return err
 	}