rlp: support for uint256 (#26898)

This adds built-in support in package rlp for encoding, decoding and generating code dealing with uint256.Int.

---------

Co-authored-by: Felix Lange <fjl@twurst.com>

rlp/rlpgen/testdata/uint256.in.txt

@@ -0,0 +1,10 @@
+// -*- mode: go -*-
+
+package test
+
+import "github.com/holiman/uint256"
+
+type Test struct {
+	Int      *uint256.Int
+	IntNoPtr uint256.Int
+}

rlp/rlpgen/testdata/uint256.out.txt

@@ -0,0 +1,44 @@
+package test
+
+import "github.com/ethereum/go-ethereum/rlp"
+import "github.com/holiman/uint256"
+import "io"
+
+func (obj *Test) EncodeRLP(_w io.Writer) error {
+	w := rlp.NewEncoderBuffer(_w)
+	_tmp0 := w.List()
+	if obj.Int == nil {
+		w.Write(rlp.EmptyString)
+	} else {
+		w.WriteUint256(obj.Int)
+	}
+	w.WriteUint256(&obj.IntNoPtr)
+	w.ListEnd(_tmp0)
+	return w.Flush()
+}
+
+func (obj *Test) DecodeRLP(dec *rlp.Stream) error {
+	var _tmp0 Test
+	{
+		if _, err := dec.List(); err != nil {
+			return err
+		}
+		// Int:
+		var _tmp1 uint256.Int
+		if err := dec.ReadUint256(&_tmp1); err != nil {
+			return err
+		}
+		_tmp0.Int = &_tmp1
+		// IntNoPtr:
+		var _tmp2 uint256.Int
+		if err := dec.ReadUint256(&_tmp2); err != nil {
+			return err
+		}
+		_tmp0.IntNoPtr = _tmp2
+		if err := dec.ListEnd(); err != nil {
+			return err
+		}
+	}
+	*obj = _tmp0
+	return nil
+}