Some rendering improvements

[mdevlamynck]

While converting my code to use elm-codegen I wanted to fix some small quirks of the rendering.

Changes in this PR:

• Hide module comment if empty
• Remove trailing lines at the end of the module
• Add a missing List.reverse to correctly order declarations in a @docs
• Add tests for hole module rendering that test the previous changes

I would also like to remove the trailing whitespace at the end of the first line of a doc comment when its empty (render {-| instead of {-|) but the change doesn't seem trivial.

[mdgriffith]

Thank you!