Implement --randomize-powerset feature

[vi]

Resolves #265.

[taiki-e]

Thanks for the PR! I'm positive on adding this feature. However:

• CI is failing, so please fix that.
  • It should be able to be fixed by running cargo fmt and adding the words listed here to .github/.cspell/project-dictionary.txt.
• Usually, I tend to choose fastrand over rand as a library for randomization unless rand is necessary for security reasons.
  • As for rand, I have experienced several maintenance pains with it in the past. (For example, the ones mentioned in Add fairness to futures_util::{select, try_select, select_ok} rust-lang/futures-rs#2641 (comment).)
  • It does not necessarily need to be done as part of this PR.
    • However, I think I need to address that before I release this feature, so if you can address that in this PR, I can publish a new release right after I merge this PR.

[vi]

Is it a good idea to also add (in this PR) a pagination options (like --powerset-start-from=3000 --powerset-num-tests=1000) for splitting big scans over multiple invocations?

[taiki-e]

As for the addition of other options, it would be easier to review if they were separated into separate PRs.

[vi]

As for the addition of other options, it would be easier to review if they were separated into separate PRs.

Implemented as a continuation PR: vi#1

Note that for some reason it includes empty feature set case in each run.