Add a not yet implemented function.

This helps when creating new parsers since todo!()s will panic the whole parser.

src/parser/util.rs

@@ -236,6 +236,13 @@ pub fn text_until_exit<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<
     ))(input)
 }
 
+#[allow(dead_code)]
+pub fn not_yet_implemented() -> Res<&'static str, ()> {
+    return Err(nom::Err::Error(CustomError::MyError(MyError(
+        "Not implemented yet.",
+    ))));
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;