Move sections to their own source file.

src/parser/headline.rs

@@ -16,8 +16,8 @@ use nom::multi::many1_count;
 use nom::multi::separated_list1;
 use nom::sequence::tuple;
 
-use super::document::section;
 use super::org_source::OrgSource;
+use super::section::section;
 use super::util::get_consumed;
 use super::util::start_of_line;
 use crate::context::parser_with_context;