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/test/util_tests.cpp

@@ -3,7 +3,6 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <clientversion.h>
-#include <fs.h>
 #include <hash.h> // For Hash()
 #include <key.h>  // For CKey
 #include <sync.h>
@@ -11,6 +10,7 @@
 #include <test/util/setup_common.h>
 #include <uint256.h>
 #include <util/bitdeque.h>
+#include <util/fs.h>
 #include <util/fs_helpers.h>
 #include <util/getuniquepath.h>
 #include <util/message.h> // For MessageSign(), MessageVerify(), MESSAGE_MAGIC