Create a `rust-toolchain.toml` using the project's MSRV

[lucamuscat]

The rust-toolchain.toml file allows a rust project to pin itself to a specific version of rust, preventing contributors from using versions of rust that are more modern than the project's Minimum Supported Rust Version (MSRV). This prevents contributors from accidentally using features introduced in more modern versions of rust that are not found in the MSRV.

Another benefit of the rust-toolchain.toml file is that invoking cargo build will automatically install and activate the rustup toolchain specified in the rust-toolchain.toml file.

I am open to contributing this PR myself if you agree to the above points.

[rainliu]

sure, please go ahead to submit a PR. Thanks.