BREAKING CHANGE

Version 2.0.0

1. Rename Map->Dict, Integer->Int, String->Str, consistent with the Python name.
2. Optimize as much as possible using standard library templates and algorithms.
3. Refactor with C++20.
4. Use Rust doc-comments style for conciseness.
5. More functionality, cleaner code, faster speed.

1. Significant performance improvements.
2. Promote String::to_integer() return type from long long to Integer.
3. Add Integer::to_integer<T>().
4. Refine doc-comments.
5. Cleaner code.

This is the last version that uses custom data structures and requires C++17.