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

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

src/blockfilter.cpp

@@ -3,6 +3,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <blockfilter.h>
+#include <crypto/siphash.h>
 #include <hash.h>
 #include <primitives/transaction.h>
 #include <script/script.h>