Fix docs and warnings

[jvoegele]

I'd like to use transform_map in one of my projects, but there were a few minor issues that I've addressed in this Pull Request.

• Fixed up the @docs so that they render properly with mix docs.
• Fixed deprecation warnings about use of Enum.into/2 with non-empty lists.
• Fixed several dialyzer warnings.
• Moved private functions to bottoms of modules, according to Elixir style conventions.

All tests still pass and docs now render correctly.