Fix #6209 - introduce standalone dir crate

* created the dir crate in util
* moved code from ethstore/src/dir/paths.rs to dir crate
* rename dir module in ethstore to accounts_dir to distinguish it
  from the dir crate
* changes after @tomusdrw on #6952

rpc/src/v1/tests/mocked/parity_accounts.rs

@@ -18,7 +18,7 @@ use std::sync::Arc;
 
 use ethcore::account_provider::{AccountProvider, AccountProviderSettings};
 use ethstore::EthStore;
-use ethstore::dir::RootDiskDirectory;
+use ethstore::accounts_dir::RootDiskDirectory;
 use devtools::RandomTempPath;
 
 use jsonrpc_core::IoHandler;