Use `expr` metavariable in `fuzz!` macro

[madsmtm]

This allows expressions not wrapped in curly brackets, which is useful when e.g. calling out to another function:

fn fuzz_with(data: &[u8]) {
    // Do fuzzing
}

fn main() {
    fuzz!(|data| fuzz_with(data));
}

// before
fn main() {
    fuzz!(|data| { // Curly bracket required.
        fuzz_with(data);
    });
}

[smoelius]

Thanks!