Implement method Slice in DataStream

[pleonex]

Similar to how Span works, we could add a method to the DataStream class to easily create a substream. We could name it Slice and it would equivalent to call the constructor with the given offset and size. This may simplify APIs.

var parent = new DataStream();
var sub = parent.Slice(4, 0x10);

Additional context
We could review the other methods that Span provides so we can have similar names.