Made test_file pass. This meant adding support for read(-1) and read()

to even the most basic file object (I also added readall() which may
be a better API).  Also, not all the tests requiring specific failure
modes could be saved.  And there were the usual str/bytes issues.
I made sure test_io.py still passes (io.py is now most thoroughly
tested by combining test_file.py and test_io.py).