Rustlings 6.2.0

Added

• Show a message before checking and running an exercise. This gives the user instant feedback and avoids confusion if the checks take too long.
• Show a helpful error message when trying to install Rustlings with a Rust version lower than the minimum one that Rustlings supports.
• Add a README.md file to the solutions/ directory.
• Allow initializing Rustlings in a Cargo workspace.
• dev check: Check that all solutions are formatted with rustfmt.

Changed

• Remove the state file and the solutions directory from the generated .gitignore file.
• Run the final check of all exercises in parallel.
• Small exercise improvements.