Implementation of BigInteger in Swift

Question

The following implementation are done in C#, and I am trying to convert into the Swift.

However, I am wondering how to use BigInteger in Swift, I have not seen this data type before. I really appreciate if anyone shares any idea to create something similar in Swift.

custom.test_u128(BigInteger.One);
CollectionAssert.AreEqual(custom.get_bytes(), new byte[] { 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 });

protected readonly MemoryStream buffer;
public byte[] get_bytes() => buffer.ToArray();