[WIP] Add XFS Filesystem Support

The following portions of the XFS code have been (at least partially) tested:
1) Superblocks
2) Allocation Groups
3) Inodes
4) Directories, including:
5) ShortForm Directory Headers and Entries
6) Single Extent Directory Headers and Entries

There is additional code in this PR that is being checked in but has not yet
been validated.  Ignore for now please.

@roliveri @Fryguy @chessbyte please review at your convenience.
Does it make sense to turn `@@fm_to_ft` into a constant `FILE_MODE_TO_FILE_TYPE_LOOKUP_TABLE`? This will quell Rubocop and remove the need for the comment above `# Lookup table for File Mode to File Type.`