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.