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/validation.cpp

@@ -19,7 +19,6 @@
 #include <consensus/validation.h>
 #include <cuckoocache.h>
 #include <flatfile.h>
-#include <fs.h>
 #include <hash.h>
 #include <kernel/chainparams.h>
 #include <kernel/mempool_entry.h>
@@ -46,6 +45,7 @@
 #include <uint256.h>
 #include <undo.h>
 #include <util/check.h> // For NDEBUG compile time check
+#include <util/fs.h>
 #include <util/fs_helpers.h>
 #include <util/hasher.h>
 #include <util/moneystr.h>