FileInfo: Add Digest and Linkname methods

[matthewrsj]

• Add a Digest method to the FileInfo struct to return the digest of the file. The digest is stored in the header at offset 1035.
• Add a ~LinkTarget~ Linkname method to the FileInfo struct to return the link target for links in the RPM. The link targets for an RPM are stored in the header at offset 1036.
• Update tests to verify new methods.

Signed-off-by: Matthew Johnson matthew.johnson@intel.com

These methods are required for use in https://github.com/clearlinux/diva. It is simple enough to implement this on our own but it would be much cleaner (and allow us to read the RPM only once) if this library exposed this information.

[matthewrsj]

Thanks for your quick response!