Add cargo msrv status (akin flutter doctor) option

[foresterre]

• [ ] Check MSRV is set and same for:
  • [ ] Cargo.toml
    • [ ] package.rust-version
    • [ ] package.metadata.msrv
  • [ ] Cargo Clippy (.clippy.toml)
  • [ ] Toolchain file
  • [ ] Rustfmt file
• [ ] Validate / parse MSRV
  • same as above
• [ ] Either package.rust-version XOR package.metadata.msrv
• [ ] If MSRV >= 1.56, and found package.metadata.msrv then package.rust-version

New sub crates (or re-use):

• [x] toolchain-file encoder / decoder
  • toml vs toml-edit
  • https://github.com/foresterre/rust-toolchain-file
• [ ] .clippy.toml encoder / decoder
  • toml vs toml-edit

Consider:

• [ ] Naming

  • [ ] cargo msrv status
  • [ ] cargo msrv info
  • [ ] cargo msrv check: Is cargo msrv check too similarly named as verify, and too close to check command nomenclature?

• [ ] Suggesting 'consider setting MSRV in [other places]' when MSRV is present in one of the supported places

Future work:

• cargo msrv fix or cargo msrv $check --fix
  • Takes highest MSRV and sets in for all places, or, runs cargo-msrv and sets found version for all?