Update `series` with canonical tests

[BNAndras]

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.

(defmodule series
  (export (slices 2)))
  
 ; Please implement the slices function