Add a 'hexdigest' class method to each digest

Add a 'hexdigest' class method to each digest algorithm; otherwise, the
Digest::hexdigest always returns the hexadecimal representation of the
digest in little-endian format.
Fix the directives for unpack in FNV64

Fix the directives for unpack in FNV64#hexdigest and FNV64#hexdigest!.
Increment the patch version