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

ethcore/src/account_provider/mod.rs

@@ -28,7 +28,7 @@ use ethstore::{
 	SimpleSecretStore, SecretStore, Error as SSError, EthStore, EthMultiStore,
 	random_string, SecretVaultRef, StoreAccountRef, OpaqueSecret,
 };
-use ethstore::dir::MemoryDirectory;
+use ethstore::accounts_dir::MemoryDirectory;
 use ethstore::ethkey::{Address, Message, Public, Secret, Random, Generator};
 use ethjson::misc::AccountMeta;
 use hardware_wallet::{Error as HardwareError, HardwareWalletManager, KeyPath, TransactionInfo};