Changes since 0.3.3:
- `b3sum` now supports the `--check` flag. This is intended to be a
  drop-in replacement for e.g. `md5sum --check` from Coreutils. The
  behavior is somewhat stricter than Coreutils with respect to invalid
  Unicode in filenames. For a complete description of how `--check`
  works, see the file `b3sum/what_does_check_do.md`.
- To support the `--check` feature, backslashes and newlines that appear
  in filenames are now escaped in the output of `b3sum`. This is done
  the same way as in Coreutils.
- To support `--check` interoperability between Unix and Windows,
  backslashes in filepaths on Windows are now replaced with forward
  slashes in the output of `b3sum`. Note that this is different from
  Coreutils.