accounts/abi: move U256Bytes to common/math (#21020)

common/math/big.go

@@ -184,6 +184,12 @@ func U256(x *big.Int) *big.Int {
 	return x.And(x, tt256m1)
 }
 
+// U256Bytes converts a big Int into a 256bit EVM number.
+// This operation is destructive.
+func U256Bytes(n *big.Int) []byte {
+	return PaddedBigBytes(U256(n), 32)
+}
+
 // S256 interprets x as a two's complement number.
 // x must not exceed 256 bits (the result is undefined if it does) and is not modified.
 //

common/math/big_test.go

@@ -212,6 +212,16 @@ func TestU256(t *testing.T) {
 	}
 }
 
+func TestU256Bytes(t *testing.T) {
+	ubytes := make([]byte, 32)
+	ubytes[31] = 1
+
+	unsigned := U256Bytes(big.NewInt(1))
+	if !bytes.Equal(unsigned, ubytes) {
+		t.Errorf("expected %x got %x", ubytes, unsigned)
+	}
+}
+
 func TestBigEndianByteAt(t *testing.T) {
 	tests := []struct {
 		x   string