core/crypto: Exile keccak, md5 and sha1 to legacy

In an perfect world these would just be removed, but the world is
imperfect, and people are forced to interact/interface with things
that are broken.

vendor/botan/legacy/README.md

@@ -0,0 +1,10 @@
+# crypto/legacy
+
+These are algorithms that are shipped solely for the purpose of
+interoperability with legacy systems.  The use of these packages in
+any other capacity is discouraged, especially those that are known
+to be broken.
+
+- keccak - The draft version of the algorithm that became SHA-3
+- MD5 - Broken (https://eprint.iacr.org/2005/075)
+- SHA-1 - Broken (https://eprint.iacr.org/2017/190)