Use static strings for CustomError.

This resulted in a 20% speedup.

src/error/error.rs

@@ -4,10 +4,9 @@ use nom::IResult;
 
 pub(crate) type Res<T, U> = IResult<T, U, CustomError<T>>;
 
-// TODO: MyError probably shouldn't be based on the same type as the input type since it's used exclusively with static strings right now.
 #[derive(Debug)]
 pub enum CustomError<I> {
-    MyError(MyError<I>),
+    MyError(MyError<&'static str>),
     Nom(I, ErrorKind),
     IO(std::io::Error),
 }
@@ -31,3 +30,9 @@ impl<I> From<std::io::Error> for CustomError<I> {
         CustomError::IO(value)
     }
 }
+
+impl<I> From<&'static str> for CustomError<I> {
+    fn from(value: &'static str) -> Self {
+        CustomError::MyError(MyError(value))
+    }
+}