implement a command about show container process.

[guni1192]

Now, $ cromwell ps is implemented by super::pids::{Pidfile, show}.
But the state of pids::Pidfile overlaps with process::Process.
I proposal follow.

pub enum State {
  Created
  Running
  Pausing
  Paused
  Stopped
}

pub enum Status {
  state: State
  pid: Pid
}

pub struct Container {
// ...
  status: Status
}

impl Container {
// ...

  // get all container process 
  fn processes() -> [Pid] {}

  // return process state
  fn status(&self) -> Status {}
}