This commit adds the System.fetch_env/1 and System.fetch_env!/1 functions as per the discussion in https://groups.google.com/forum/#!topic/elixir-lang-core/QGxEibK4y7Q. I chose to keep using the same underlying implementation for fetch_env/1 as the one for get_env/1 because the number of lines would have been the same but we would have added one function call.