Update from structopt to clap v3

[Sciencentistguy]

Clap v3 has merged structopt directly into clap, and structopt itself is now in maintenance-only mode.

This PR switches all use of structopt over to clap's new #[derive(Parser)].

[codecov]

Codecov Report

Merging #227 (90ecf8e) into master (502c8f3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #227   +/-   ##
=======================================
  Coverage   87.02%   87.02%           
=======================================
  Files          17       17           
  Lines        2381     2381           
=======================================
  Hits         2072     2072           
  Misses        309      309           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 502c8f3...90ecf8e. Read the comment docs.

[jonhoo]

Thanks for taking the time to do this! It looks like this ends up bumping the MSRV, which I think is okay for this change, but we should update CI to match the new required version 👍

inferno/azure-pipelines.yml

Line 5 in 502c8f3

minrust: 1.46.0

[jonhoo]

Excellent, thanks!

[jonhoo]

Actually, before I merge, cc @djc for cargo-flamegraph to see how problematic this would be there. I think the move to clap3 is pretty worthwhile.

[djc]

Should be fine, we already ported flamegraph (which now disables the cli feature in inferno). I'm discussing the MSRV issue upstream.