Move the sexp parser into the parser crate.

We need to parse sexp for diary sexp.

build.rs

@@ -61,7 +61,7 @@ fn write_header(test_file: &mut File) {
 use organic::compare_document;
 use organic::parser::document;
 use organic::emacs_parse_org_document;
-use organic::sexp;
+use organic::parser::sexp::sexp;
 
 "#
     )