Use `flutter_rust_bridge`, the high-level memory-safe binding generator, to avoid manipulating with pointers manually to enhance this library

[fzyzcjy]

Hi thanks for this wonderful starter! As shown in the sample code of this repo, we have to manually write down allocations/frees for pointers such as Strings. That is still quite error-prone and cumbersome. So maybe we can use flutter_rust_bridge, which will automatically generate all such bindings.