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/i2p.cpp

@@ -6,13 +6,13 @@
 #include <compat/compat.h>
 #include <compat/endian.h>
 #include <crypto/sha256.h>
-#include <fs.h>
 #include <i2p.h>
 #include <logging.h>
 #include <netaddress.h>
 #include <netbase.h>
 #include <random.h>
 #include <tinyformat.h>
+#include <util/fs.h>
 #include <util/readwritefile.h>
 #include <util/sock.h>
 #include <util/spanparsing.h>