Port to clap v3

[rsdy]

This is a reasonably straightforward port of Abscissa to Clap v3 using the custom derives.
I removed most of the command subsystem (EntryPoint, Usage, Help) since Clap takes over most of those roles.

To demonstrate how this could work, I also migrated the CLI.

The ergonomics might be improved, as subcommands work differently in Clap than Gumdrop. This is best seen in the commands.rs file of the CLI and gen.rs root command. Embedding subcommands can get tedious quickly.

Another gotcha I found is that #[derive(Runnable)] will not work for structs in its current form, so a manual implementation is necessary.

If this changeset looks like a viable approach, I can take a look at porting the template.

Resolves #298

[tony-iqlusion]

Thanks! This is definitely wanted and we're glad you opened the PR!

Per the clap v3 docs:

https://crates.io/crates/clap/3.0.0-beta.2

We have a 3.0.0-beta.2 prerelease out but we do not give any guarantees that it's API is stable. We do not have a changelog yet which will be written down after we are sure about the API stability. We recommend users to not update to the prerelease version yet and to wait for the official 3.0.

We definitely want to merge a PR like this after the official 3.0 release, but per that message, we don't want to do it beforehand.

[rsdy]

@tony-iqlusion Happy to keep ironing out ergonomics issues and templates until Clap stabilises.

[rsdy]

I've ported forward all the templates, and tests pass. I am extremely grateful for the test suite!

I've tried re-introducing an EntryPoint so that the application template doesn't need to deal with config and verbose, but nether trait objects or generics will work due to the Clap derive macro. Overall, having that code generated might slightly increase flexibility, but at the cost of more noise.

[tony-iqlusion]

Closing in favor of #562, which builds upon this work.