Move the sexp parser into the parser crate.

We need to parse sexp for diary sexp.

src/compare/diff.rs

@@ -1,6 +1,6 @@
-use super::sexp::Token;
 use super::util::assert_bounds;
 use super::util::assert_name;
+use crate::parser::sexp::Token;
 use crate::parser::Clock;
 use crate::parser::Comment;
 use crate::parser::CommentBlock;