Implement a digest method

The digest method returns a hexadecimal string digest.  This was an
80/20 optimization in line with the Tiny philosophy to keep things
simple and return values comparable to command line tools and what
people would see on a website with a digest to verify a file.

The default algorithm is 'SHA-256' or Digest module constructors may be
given as arguments.

Those with more specific needs should just use Digest directly.
Add test for digest method for MD5 and SHA-256
create digest test file in temp directory
Add Digest and Digest::SHA prereqs

Perl 5.10.0 is fine, but earlier Perls need versions
that support SHA-256.  With prereqs added, I removed
the skip logic from the digest tests.
update Changes, stopwords and meta files
fix up See Also and contributor mail mapping
bump Changes