new fixes for lexing process, tests to go with them

tests/test_lex.rs

@@ -58,6 +58,34 @@ mod lex_tests {
         }
     }
 
+    #[test]
+    fn test_list_delim_in_str() {
+        let document: &str = "('(')";
+        match lex(document.to_string()) {
+            Ok(tree) => {
+                assert_eq!(ast_to_string(tree), document);
+            },
+            Err(s) => {
+                print!("{}\n", s);
+                assert!(false);
+            }
+        }
+    }
+
+    #[test]
+    fn test_empty_string() {
+        let document: &str = "('')";
+        match lex(document.to_string()) {
+            Ok(tree) => {
+                assert_eq!(ast_to_string(tree), document);
+            },
+            Err(s) => {
+                print!("{}\n", s);
+                assert!(false);
+            }
+        }
+    }
+
     #[test]
     fn test_unmatched_list_delim_flat() {
         let document: &str = "(one two";