Move the sexp parser into the parser crate.

We need to parse sexp for diary sexp.

src/compare/mod.rs

@@ -1,7 +1,5 @@
 mod diff;
 mod parse;
-mod sexp;
 mod util;
 pub use diff::compare_document;
 pub use parse::emacs_parse_org_document;
-pub use sexp::sexp;