Maybe: require one of a set of features in --feature-powerset

[CAD97]

As an example, consider a crate which uses a hashmap, and has a feature std to use the std hashmap and a feature hashbrown to use the hashmap from hashbrown. This crate requires that at least one of std or hashbrown are used, so --feature-powerset won't work as-is, because it will want to test the noncompiling --no-default-features configuration. There are other feature flags which the library would like to test as well, so --exclude-no-default-features isn't sufficient.

Currently, the crate can use two runs of cargo hack, e.g.

cargo hack --feature-powerset --features=std check
cargo hack --feature-powerset --features=hashbrown check

If there's more than one such set of "require one of." you can see how this could get complicated quickly, basically being its own separate powerset.

cargo-hack could potentially provide a flag --required-features-group std,hashbrown with the semantics of discarding any feature set which does not include at least one of the listed features in order to make this situation easier to test.

[taiki-e]

I would accept a PR to add this. (This appears to be a more manageable version of #115)