Fallible visitor methods

[tigerros]

Makes Visitor methods fallible (except for end_game). You can see why this is useful in action in the examples (particularly the new one, extract_fen). It makes error handling for users a lot cleaner.

Users who don't need error handling can simply use Infallible as the error type.

What happens after an error

The reader continues to parse the game even if the visitor errors, it just doesn't call any visitor methods afterwards. Early returns would be nice, but they corrupt the reader/split games. This means that there's no extra functionality in this PR, just a nicer API.

Example

Here's an example of an old way of handling fallible visitors:

struct NoFen(Result<NoFenInner, NoFenError>);

impl Visitor for NoFen {
    type Result = Result<NoFenOutput, NoFenError>;

    fn tag(&mut self, name: &[u8], _: RawTag<'_>) {
        let Ok(inner) = self.0 else { return; };

        if name == b"FEN" {
            self.0 = Err(NoFenError { ... });
            return;
        }

        // ...
    }

    fn san(&mut self, san: SanPlus) {
        let Ok(inner) = self.0 else { return; };
        // ...
    }

    fn end_game(&mut self) -> Self::Result {
        match self.0.as_mut() {
            Ok(inner) => Ok(inner.output()),
            // assuming NoFenError is not Copy
            Err(e) => Err(std::mem::replace(e, NoFenError::default())),
        }
    }
}

Alternative might be to store Option<NoFenError> and merge NoFenInner into NoFen.

Here's equivalent code with this PR:

struct NoFen { ... }

impl Visitor for NoFen {
    type Output = NoFenOutput;
    type Error = NoFenError;

    fn tag(&mut self, name: &[u8], _: RawTag<'_>) -> Result<(), Self::Error> {
        if name == b"FEN" {
            return Err(NoFenError { ... });
        }
        
        // ...
        Ok(())
    }

    fn san(&mut self, san: SanPlus) -> Result<(), Self::Error> {
        // ...
        Ok(())
    }

    fn end_game(&mut self) -> Self::Output {
        // output directly
    }
}

As you can see, it's a lot nicer.

[tigerros]

Moved to niklasf/shakmaty#104