Refactor CLI argument parsing in crate_extractor

[m-bock]

• Updates CLI parsing library "clap" from 2.3 to 3.0

  • In this latest version it's encouraged to use derive macros for parser generation
    The advantage is:
    • A clear rust data structure is defined to contain the CLI arguments
    • parser is fully derived/generated from that
    • Parsing and data usage are separated much better ("Parse, don't validate" principle)

• Cli argument parsing is outsourced in "cli.rs"

• Introduced constants for default arguments

• Makes CI available on all branches

[achimcc]

Nice, thanks! This also solves @athei last wish from my previous PR #25, since the latest version of clap seems to suppport the same features as structopt?

[athei]

Yes. It seems that the new clap version obsoletes structopt.

[m-bock]

@athei
Cool, it's still in beta but worked out really well. Yesterday we tried to find something similar for environment variable parsing. we found envy. Unfortunately it's a bit wordy to define default arguments: https://github.com/paritytech/ink-playground/blob/main/crates/backend/src/env.rs#L17
It's ok, but would be nicer to find something that handles defaults like clap. Do you maybe know about something?

[athei]

I think clap supports reading default values from the environment. Why do you need a second crate?

[m-bock]

Oh, we did not see this option. But now I found it.
Thanks for the hint, I think we'll change this!