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/fuzz/fuzz.cpp

@@ -4,11 +4,11 @@
 
 #include <test/fuzz/fuzz.h>
 
-#include <fs.h>
 #include <netaddress.h>
 #include <netbase.h>
 #include <test/util/setup_common.h>
 #include <util/check.h>
+#include <util/fs.h>
 #include <util/sock.h>
 #include <util/time.h>