fully independent IEx.Debugger shell

* Every shell command starts a new process, and results come with
  the corresponding pid.
* Current issues/TODO:
  - Handle incomplete input.
  - Breakpoints not working, as __FILE__ from compiled modules
    being replaced by "iex".
  - `ds` helper currently recieves a quoted expression.
    This function should start a full shell inside the process if
    possible.
  - Missing step and pause helpers.