[elixir] Impelement ListOps.append w/ passing tests
[elixir] Implement what I think *should* be ListOps.concat

Unfortunately, the ["concat of huge list of small lists" test][t] is
never completing. I'm not quite sure what I'm doing wrong.

[t]: https://github.com/andrewsardone/exercism-exercises/blob/708cbb7040169c1cb2bac9a8c3d8d897f2dbe6aa/elixir/list-ops/list-ops_test.exs#L112-L115