Optional std in optional dependencies

[dhardy]

#106 makes std optional.

To quote myself (to create an open issue):

I foresee an issue here: rand_core has optional dependencies on getrandom and std, and now needs to depend on getrandom/std only if both are enabled. I'm not sure how to do that. See here where the same issue came up recently.

This is really a Cargo issue IMO, but something we may have to find a workaround to.

[josephlr]

Is this an issue if the rand_core's std feature unconditionally enables getrandom? That seems to be what it does currently.

Also, #106 doesn't actually change much regarding this. std was already optional for many targets on getrandom (like Windows) before that change. The PR just makes the feature act consistently.

[newpavlov]

No, the problem is that rand_core uses getrandom's std feature for converting getrandom::Error into Box<dyn std::error::Error>. We want to enable std feature for getrandom only if both getrandom and std features are enabled for rand_core, but unfortunately AFAIK it's impossible to define such relation right now, since std = ["getrandom/std"] will enable getrandom optional dependency. So we either can't enable std feature for getrandom or we always depend on getrandom with an enabled std feature.

But actually it does not matter for rand_core v0.5, since it always enables getrandom with an enabled std feature.