For a file f, iter(f) now returns f (unless f is closed), and f.next()
is similar to f.readline() when EOF is not reached; however, f.next()
uses a readahead buffer that messes up the file position, so mixing
f.next() and f.readline() (or other methods) doesn't work right.
Calling f.seek() drops the readahead buffer, but other operations
don't.

The real purpose of this change is to reduce the confusion between
objects and their iterators.  By making a file its own iterator, it's
made clearer that using the iterator modifies the file object's state
(in particular the current position).

A nice side effect is that this speeds up "for line in f:" by not
having to use the xreadlines module.  The f.xreadlines() method is
still supported for backwards compatibility, though it is the same as
iter(f) now.

(I made some cosmetic changes to Oren's code, and added a test for
"file closed" to file_iternext() and file_iter().)