feat: Use RPITIT instead of Box<dyn ...>

[mlgiraud]

Just a small PR to replace Box<dyn ...> with RPITIT, since it is now stable. I believe this is what your todo in the trait was for?

[mlgiraud]

Maybe it would also make sense to return a type that implements the Error trait, instead of something that just implements Debug?

[mlgiraud]

Another question maybe: Why does the fetch function have to take a mutable reference to self? Shouldn't this be an immutable function?

EDIT: This makes it practically impossible to have the parser borrow data from an input cache, and also pass the input cache to the write function for the error report, since this would require a mutable borrow of the input cache, when the input cache is already borrowed read only for the parsed data.