Replace structopt dependency by clap

[Expyron]

StructOpt features have been integrated into clap v3, and it is now in maintenance mode.
The maintainer recommends migrating away from it: TeXitoi/structopt#525

This touches argument parsing for cargo fmt. The behavior was kept the same.

There are visual differences in the cargo fmt --help output.

cargo fmt --help before:

This utility formats all bin and lib files of the current crate using rustfmt.

USAGE:
    cargo fmt [FLAGS] [OPTIONS] [-- <rustfmt_options>...]

FLAGS:
        --check      Run rustfmt in check mode
        --all        Format all packages, and also their local path-based dependencies
    -h, --help       Prints help information
    -q, --quiet      No output printed to stdout
    -v, --verbose    Use verbose output
        --version    Print rustfmt version and exit

OPTIONS:
        --manifest-path <manifest-path>      Specify path to Cargo.toml
        --message-format <message-format>    Specify message-format: short|json|human
    -p, --package <package>...               Specify package to format

ARGS:
    <rustfmt_options>...    Options passed to rustfmt

cargo fmt --help after:

This utility formats all bin and lib files of the current crate using rustfmt.

USAGE:
    cargo fmt [OPTIONS] [-- <rustfmt_options>...]

ARGS:
    <rustfmt_options>...    Options passed to rustfmt

OPTIONS:
        --all
            Format all packages, and also their local path-based dependencies

        --check
            Run rustfmt in check mode

    -h, --help
            Print help information

        --manifest-path <manifest-path>
            Specify path to Cargo.toml

        --message-format <message-format>
            Specify message-format: short|json|human

    -p, --package <package>...
            Specify package to format

    -q, --quiet
            No output printed to stdout

    -v, --verbose
            Use verbose output

        --version
            Print rustfmt version and exit

[calebcartwright]

Thank you for this. We discussed this at a dev tools meeting a couple months back so I've had this on my radar but obviously hadn't gotten around to it.

Working towards prepping the next release so probably going to wait to review and merge this till afterwards

[calebcartwright]

Upstream changes from the subtree in r-l/rust have been pulled in and as anticipated there's now merge conflicts. Would you mind rebasing and resolving any conflicts when you get a chance?

[Expyron]

Rebased and resolved the conflicts in Cargo.lock/Cargo.toml

[ytmimi]

Changes look good to me. Thanks for rebasing and fixing merge conflicts!

[calebcartwright]

Thanks so much!