Add XFS Filesystem Support

The following portions of the XFS code have been tested:
1) Superblocks
2) Allocation Groups
3) Inodes
4) Directories, including:
 * ShortForm Directory Headers and Entries
 * Single Extent Directory Headers and Entries
 * Single Level BTree Directory Headers and Entries

Unused code has been removed.
Additional updates based on Rubocop warnings have been made.
Additional updates based on @Fryguy comments have been made - mostly based on
file naming conventions.
More testing is needed before release including the find test being written
by @roliveri.

@roliveri @Fryguy @chessbyte please review at your convenience.
Add XFS Filesystem Support

Code to support Smart State Analysis of XFS filesystems has been
added.  The following has been tested successfully:
1) Superblocks
2) Allocation Groups
3) Inodes
4) Directories, including:
5) ShortForm Directory Headers and Entries
6) Single Extent Directory Headers and Entries
7) Multi-block Extent Directory Headers and Entries
8) Single-level BTree Directory Headers and Entries
9) Multi-level BTree Directory Headers and Entries
10) Single and multi-block extent regular files.
11) Symbolic Links

A fleece test on a local filesystem has been run successfully.
Changes due to relevant RuboCop warnings have been added as well
as those in response to review comments.

@chessbyte @Fryguy @roliveri please review as appropriate.
Merge pull request #490 from jerryk55/xfs_filesystem

Add XFS Filesystem Support