Fix compile warnings

[nsarno]

I get several warning during compilation with Elixir 1.3.2. Most of them are unused variables or typos.

Changes

• Prefix unused variables with _. Some of them could be removed entirely but kept them with a prefix for context.
• Fix obvious typos.
• Replace deprecated modules/functions.

  Notes

• I assumed Lists.member?/2 was supposed to be Enum.member?
• I couldn't find anything about Set.from_list/2. I replaced it with MapSet.new/2 but that's only a guess.
• There seem to be missing tests around the code from the last 3 commits. I'm lacking too much understanding of the codebase to be able to add them myself.

Hope this helps! Cheers

[aeden]

Thanks for the PR!