- The doctests in decistmt() weren't run at all when
  test_tokenize was run via regrtest.py.

- Some expected output in decistmt() was Windows-specific
  (but nobody noticed because the doctests weren't getting
  run).

- test_roundtrip() didn't actually test anything when
  running the tests with -O.  Now it does.

- Changed test_roundtrip() to show the name of the input
  file when it fails.  That would have saved a lot of
  time earlier today.

- Added a bunch of comments.