require scalar key arrays when seeding, splitting, and folding
require scalar key arrays when seeding, splitting, and folding

Recent changes to RNG internals actually make it easier for us to
render these operations batch-polymorphic. However, any existing use
of these in a non-scalar way suggests incorrect usage, since they were
scalar-only before (albeit imperfectly guarded as such).

google/jax,google/jax,google/jax,google/jax