chore: add test with custom type

src/std/option.rs

@@ -16,6 +16,7 @@ where
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::structs::Pair;
 
     #[test]
     fn should_parse_empty_string_as_none() {
@@ -34,4 +35,13 @@ mod tests {
             _ => unreachable!(),
         }
     }
+
+    #[test]
+    fn should_parse_pair() {
+        let estr = EString::from("1+2");
+        match estr.parse::<Option<Pair<i32, '+', i32>>>() {
+            Ok(res) => assert_eq!(res, Some(Pair(1, 2))),
+            _ => unreachable!(),
+        }
+    }
 }