Limit refresh rate of download progress bar to once per second

See merge request !42

This should significantly reduce verbosity of CI logs.

Replace ordermap with indexmap

See merge request !39

Fix description deprecation warnings

See merge request !38

The `description` and `cause` methods in `Error` were soft deprecated in Rust 1.27 and hard deprecated in Rust 1.42 [[1]]. This change removes the use of these deprecated methods and fixes three deprecation warnings.

You can verify that the test output has not changed with the following steps:

1. Create the file `/etc/pkg.d/localtest`
2. Add the line `localhost:1234` to `/etc/pkg.d/localtest`
3. Run Netcat on port 1234 in a new terminal to simulate a server:
    ```console
    $ while true; do { echo -e 'HTTP/0 200 OK\r\n'; } | nc -l 1234; done
    ```
    The server returns an invalid HTTP version on purpose.
4. Try to fetch a package, e.g. `pixelcannon`:
   ```console
   $ cargo run -- fetch pixelcannon
   ```
5. Notice that you get the following error message with or without the changes:
   ```console
   * Requesting http://localhost:1234/x86_64-unknown-linux-gnu/pixelcannon.sig
   pkg: fetch: pixelcannon: failed: Invalid HTTP version specified
   ```

[1]: https://blog.rust-lang.org/2020/03/12/Rust-1.42.html#errordescription-is-deprecated

Update documentation

See merge request redox-os/pkgutils!37

I've made the following changes to make pkg more user-friendly:
    - Added an 'about' string to every subcommand (keeping the old help
      messages that were present before clap)
    - Added a basic description to the application
    - Reformatted some of the code due to rustfmt

Use Rust 2018 edition

See merge request redox-os/pkgutils!36

Update Cargo.lock

See merge request redox-os/pkgutils!33

AArch64 builds need a newer syscall crate.

update to use liner fork