Add rustfmt.toml to imports_granularity and group_imports option

[nabetti1720]

Hello Contributors.

Previously, in PR #328, we have implemented the addition of settings to rustfmt.toml by @yollotltam.

At that time, it seems that the following settings were not applied because of nightly.

• unstable_features = true
• imports_granularity = "Crate" https://rust-lang.github.io/rustfmt/?version=v1.7.0&search=#imports_granularity
• group_imports = "StdExternalCrate" https://rust-lang.github.io/rustfmt/?version=v1.7.0&search=#group_imports

However, lllrt has already applied nightly build to the toolchain and does not see the addition of these settings as a barrier. Of course, it would have to be run with cargo +nightly fmt.

The repeated addition of features has led to poor visibility around the use instruction. And I seem to have caused that too.

I would like to add the above settings to improve the efficiency and visibility of adding the use instruction, please let me know your thoughts.

[richarddavison]

I'm not apposed to adding this as long as we still can run on stable. It's important for us that the project still builds and runs on non-nightly toolchains.

[nabetti1720]

Thank you very much. I will think about it again when I get to stable.