Allow multiple parameters to be passed to `fuzz_target` macro

[frewsxcv]

I'm currently doing something like this to work around the one parameter limitation:

fuzz_target!(|tuple: (geo_types::Polygon<f32>, f32)| {
    let (polygon, epsilon) = tuple;

    let simplified = polygon.simplify(&epsilon);

    check_result(polygon, simplified);
});

[fitzgen]

Yeah, we could have the macro do this tuple dance for you, for sure.

[fitzgen]

Going to close this in favor of rust-fuzz/libfuzzer#77, since that issue is in the right repo.