Replace all md5 examples w/ secure examples

[John-Mark Gurney <jmg@newcontext.com>]

There are many examples using md5, such as in Part 3, Section 2.5 example.  MD5 has long been known to be insecure and using it in examples will have some people think that it is acceptable practice to use these hashes in contexts where they should not be (such as in a whitelist instead of a blacklist, which some AV vendors have done, even today).

I would recommend replacing them w/ one of SHA-256, SHA-512/256 or SHA3-256.

Thanks.

John-Mark