kernel: Move BlockInfo to a kernel file

This should avoid having to include interfaces/chain.h from a kernel
module. interfaces/chain.h in turn includes a bunch of non-kernel
headers, that break the desired library topology and might introduce
entanglement regressions.

src/kernel/chain.cpp

@@ -2,9 +2,10 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
-#include <chain.h>
-#include <interfaces/chain.h>
 #include <kernel/chain.h>
+
+#include <chain.h>
+#include <kernel/cs_main.h>
 #include <kernel/types.h>
 #include <sync.h>
 #include <uint256.h>