[CBOR] Implement indefinite length writer and reader support

[eiriktsarpalis]

Adds indefinite length writer and reader support for strings, maps and arrays:

• Implement write support.
• Implement read support.

[bartonjs]

I don't know if it's worth the complexity; but you could do something like

List<(int offset, int length)> ranges = Interlocked.Exchange(ref _ranges, null);
ranges ??= new List<(int, int)>();
ranges.Clear();

And have all of the calling members assign it back to the field right before returning.

It's not super important, though.

[eiriktsarpalis]

I'll give it a shot, provided it doesn't make things too hard to read. Or maybe wait until benchmarking demonstrates issues with allocations.

Also, why Interlocked? The class offers no thread safety guarantees.

[bartonjs]

It's both a quick way of "I've taken this" (in case somehow this ever went recursive, which I know isn't currently possible) and it means that the kinds of scary race conditions are limited. (overlapping writes from a read method is scarier to me than read twice or read slightly forward)