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/interfaces/wallet.h

@@ -6,11 +6,11 @@
 #define BITCOIN_INTERFACES_WALLET_H
 
 #include <consensus/amount.h>
-#include <fs.h>
 #include <interfaces/chain.h>          // For ChainClient
 #include <pubkey.h>                    // For CKeyID and CScriptID (definitions needed in CTxDestination instantiation)
 #include <script/standard.h>           // For CTxDestination
 #include <support/allocators/secure.h> // For SecureString
+#include <util/fs.h>
 #include <util/message.h>
 #include <util/result.h>
 #include <util/ui_change_type.h>