Enable multi-threaded formatting for `cargo fmt --all` to speed up large projects

[eval-exec]

Problem

It seems that cargo fmt --all only use 1 core, if the cargo project has many .rs file, cargo fmt --all is slow, may take several seconds or minutes. I think we should refactor cargo fmt --all to use multi-threads.

Proposed Solution

No response

Notes

No response

[eval-exec]

I can resolve this issue—if everyone agrees that cargo fmt --all should support multi-threading, please assign the issue to me.

[weihanglo]

While being an official Cargo command, cargo fmt is technically an external Cargo command not bundled together in the cargo binary. cargo-fmt is developed in https://github.com/rust-lang/rustfmt. Let me transfer this there.

@rustbot transfer rustfmt

[ehuss]

I believe this is a duplicate of #6091.