The logic to generalize over downloading a URL or reading data from
the file-system included static values in the iterator along with the
data being read - so the caller could access the total expected size of
the data being read. While this worked, the result didn't read well.
Now these iterators yield the data being read, a new type has been
added for the caller to read the size hint from.
Also correct invalid doc-string.
Campbell Barton
3ee045724f