Improve the debug output a little
Simplify `elixir-smie-next-token-no-lookaround` a bit.

This pulls the "newline as whitespace is significant" logic up into
`elixir-smie-next-token` and refactors the comment/whitespace-skipping
logic a bit to deal with the possibility of skipping newlines.
Add some more tests (including failing ones for #5).