slice_partition_at_index is no longer supported

[krojew]

Common issues

Problem: rand with nightly feature has been broken for some time, since slice_partition_at_index had been removed.

Quick solution: Don't use nightly feature or remove slice_partition_at_index.

Details: slice_partition_at_index was removed recently, thus the nightly breaks the build at https://github.com/rust-random/rand/blob/master/src/lib.rs#L53.

[TheIronBorn]

It seems we aren't even using the feature because we switched to select_nth when the name changed. We could probably just remove the feature without concern