Fixes #235 - Replace unmaintained structopt crate by clap

[palant]

See #235 for the rationale. The change itself is pretty straightforward, the only issue here: existing code will have to be adapted (typically a trivial structopt::StructOpt => clap::Parser and Opt::from_args => Opt::parse search&replace).

A bunch of tests are failing for me, but that’s unchanged from current main branch.

[palant]

Concerning the clippy warning I fixed: that’s unrelated to my change. The only reason this issue wasn’t flagged on main is that the check was running Rust 1.77.2 there, and this warning is new in Rust 1.78.

[johnhurt]

Awesome. We merged this change internally, but had to go to an clap 3.x to maintain our minimum rust version. It will show up in the next internal -> external sync. Thanks!

[drcaramelsyrup]

Thanks for contributing! This was merged into main in 795594d (aforementioned adjustment in 9dee8e6).