fix: make small fixes

[silathdiir]

Summary

• add toolchain.toml to set nightly rust.
• update .gitignore.
• simplify find_index_subvector function, and tested as:

fn main() {
    let v = vec![1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
    let t = &[5, 6, 7];

    let r = v.windows(t.len()).position(|s| s == t);
    // Result is `Some(4)`.
    println!("Found result = {r:?}");
}

[nikkolasg]

🙏 Thank you !