Manage memory in the language itself

[temeddix]

Changes

Before, Rust was in charge of freeing the memory allocated from Dart. However, for extra-clear memory safety, freeing the memory from Dart can be better.

• https://doc.rust-lang.org/std/vec/struct.Vec.html#method.from_raw_parts

Before Committing

Please make sure that you've analyzed and formatted the files.

dart analyze flutter_ffi_plugin --fatal-infos
dart format .
cargo fmt
cargo clippy --fix --allow-dirty