#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.