Move the sexp parser into the parser crate.

We need to parse sexp for diary sexp.

src/lib.rs

@@ -7,8 +7,6 @@ mod compare;
 pub use compare::compare_document;
 #[cfg(feature = "compare")]
 pub use compare::emacs_parse_org_document;
-#[cfg(feature = "compare")]
-pub use compare::sexp;
 
 mod error;
 pub mod parser;