Simplify switch statement in `parse_statements`.

[p8]

:on_nl, :on_ignored_nl, :on_comment, :on_embdoc are handled by a single when branch. In this branch there are multiple if branches handling :on_nl and :on_ignored_nl separately from :on_comment and :on_embdoc.

By having separate when branches for :on_nl/:on_ignored_nl and :on_comment/:on_embdoc we can remove the if statements.