lnwire: export ReadElements and WriteElements

In this commit, we export the ReadElements and WriteElements functions.
We do this as exporting these functions makes it possible for outside
packages to define serializations which use the BOLT 1.0 wire format.

lnwire/lnwire_test.go

@@ -175,7 +175,7 @@ func TestMaxOutPointIndex(t *testing.T) {
 	}
 
 	var b bytes.Buffer
-	if err := writeElement(&b, op); err == nil {
+	if err := WriteElement(&b, op); err == nil {
 		t.Fatalf("write of outPoint should fail, index exceeds 16-bits")
 	}
 }