Extract CSipHasher to it's own file in crypto/ directory.

This is a move-only commit with the exception of changes to includes.

src/test/hash_tests.cpp

@@ -2,6 +2,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <crypto/siphash.h>
 #include <hash.h>
 #include <util/strencodings.h>
 #include <test/test_bitcoin.h>