Add support for Stuffit SpaceSaver formats

[ci-macpaw]

Originally reported on Google Code with ID 300

d235j.1:

Stuffit SpaceSaver formats are not implemented. This was used for transparent compression.
The software was shipped with StuffIt Deluxe 3 through 5 (or 6).

Some testcases: http://dec8.info/priv/spacesaver_testcases/ (version 3.5 so far)

There seem to be two variants, a fast and a maximum, but I can't seem to get both to
be created. :(