Use cargo and rustc 1.60 or later.

  - Don't use the `rust-toolchain` file to specify the version of
    `cargo` and `rustc` to use.  Instead use `rust-version` in
    `Cargo.toml`.

  - Note: this changes how we choose the tool chain: `rust-version` is
    a minimal version; `rust-toolchain` is an exact version.

  - Use 1.60 as that is what is currently available in Debian testing.

  - Closes #7, #10.