Move the sexp parser into the parser crate.

We need to parse sexp for diary sexp.

src/compare/util.rs

@@ -1,7 +1,6 @@
+use crate::parser::sexp::Token;
 use crate::parser::Source;
 
-use super::sexp::Token;
-
 /// Check if the child string slice is a slice of the parent string slice.
 fn is_slice_of(parent: &str, child: &str) -> bool {
     let parent_start = parent.as_ptr() as usize;