rlp: add Flat

rlp/encode_test.go

@@ -177,6 +177,15 @@ var encTests = []encTest{
 	{val: &recstruct{5, nil}, output: "C205C0"},
 	{val: &recstruct{5, &recstruct{4, &recstruct{3, nil}}}, output: "C605C404C203C0"},
 
+	// flat
+	{val: Flat(uint(1)), error: "rlp.Flat: uint did not encode as list"},
+	{val: Flat(simplestruct{A: 3, B: "foo"}), output: "0383666F6F"},
+	{
+		// value generates more list headers after the Flat
+		val:    []interface{}{"foo", []uint{1, 2}, Flat([]uint{3, 4}), []uint{5, 6}, "bar"},
+		output: "D083666F6FC201020304C2050683626172",
+	},
+
 	// nil
 	{val: (*uint)(nil), output: "80"},
 	{val: (*string)(nil), output: "80"},