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

parity/cli/usage.rs

@@ -154,7 +154,7 @@ macro_rules! usage {
 		use std::io::{Read, Write};
 		use parity_version::version;
 		use clap::{Arg, App, SubCommand, AppSettings, ArgMatches as ClapArgMatches, Error as ClapError, ErrorKind as ClapErrorKind};
-		use helpers::replace_home;
+		use dir::helpers::replace_home;
 		use std::ffi::OsStr;
 		use std::collections::HashMap;