Update clap to 3.1

[Enet4]

I was unable to build the upstream branch, so I took the liberty of updating Clap to make it work.

• Bump clap to 3.1.9
  • feature "derive" needed for Parser derive macro
  • feature "cargo" required for crate_authors and other macros
  • feature "env" required for env field in clap attribute
• Replace deprecated cargo_generate crate with cargo_complete
• Replace deprecated constructs according to suggestions