refactor: Move fs.* to util/fs.*

The fs.* files are already part of the libbitcoin_util library. With the
introduction of the fs_helpers.* it makes sense to move fs.* into the
util/ directory as well.

src/node/chainstate.cpp

@@ -16,7 +16,7 @@
 #include <tinyformat.h>
 #include <txdb.h>
 #include <uint256.h>
-#include <fs.h>
+#include <util/fs.h>
 #include <util/time.h>
 #include <util/translation.h>
 #include <validation.h>