As already mentioned within #22387 the all-new Span should bring many benefits to BigInteger.

• We currently use pointers to spare some index calculations (performance) and "abstract" the differences between ordinary arrays and stackalloc based stuff --> use spans instead
• We currently copy the array containing the actual number after every operation to trim leading zeros --> use span.slice instead?
• And maybe there's more...

We cannot get grid of unsafe code completely since there are stack-vs-heap paths based on stackalloc, but all the pointer based stuff should gracefully disappear; I think.

I'm interested to dive into this, after the summer or so.