Update `series` with canonical tests

The series exercise  was ported nine years ago and is missing several canonical tests from the problem-specifications repo. The `example.lfe` will need to be rewritten and the `series.lfe` stub will need minimal code added (see below) to define the module for the exercise.

```lfe
(defmodule series
  (export (slices 2)))
  
 ; Please implement the slices function
 
 ```