switch dfVFS to use alternative FAT and exFAT implementation instead of libtsk

[joachimmetz]

The libtsk FAT and exFAT implementations have issues:

• gaps in format support https://github.com/py4n6/pytsk/issues/82
• serious code weaknesses https://github.com/sleuthkit/sleuthkit/pulls?q=is%3Apr+author%3Ajoachimmetz+

Look for / create a more reliable alternative

• [x] complete initial version of libfsfat/pyfsfat https://github.com/libyal/libfsfat/issues/1
• [x] add back-end https://github.com/log2timeline/dfvfs/pull/672
• [ ] add support for codepages - https://github.com/log2timeline/dfvfs/issues/114
• [ ] switch default FAT back-end

[joachimmetz]

Related https://github.com/log2timeline/dfvfs/issues/356

[joachimmetz]

Changes to add experimental FAT-12, FAT-16, FAT-32 and exFAT back-end in https://github.com/log2timeline/dfvfs/pull/672