Idiomatic way of splitting iterator of Result into two iterators (Ok, Err)

[Jayshua]

@scottmcm

What has to happen when you call err_iter.next()? Well, it has to go through the iterator until it finds the first error. There might not be any, so it might have to iterate the whole thing to know to return None.

It is acceptable for an Iterator to resume iteration after returning None. It would be valid for an implementation of let (ok_iter, err_iter) = result_iter.split_ok_err(); to return two iterators that yield None until the next value in the source iterator matches their expected Result variant. That implementation would only require enough memory to store a single Result<T, E>, by using .peek() on the source iterator for example.

Here's a possible implementation.