Migrate from `structopt` to `clap` v3 (#1293)

`clap` v3 added support for a `structopt`-like parser derive usage.
`structopt` is now in maintenance mode
(https://docs.rs/structopt/latest/structopt/#maintenance); it is
recommended to migrate to `clap`.

Structs to parse CLI arguments have been renamed to `Args` for
consistency.