Rules for `kron`

[ToucheSir]

Zygote currently punts this to AD, but that trick no longer works on nightly because kron(::AbstractVector, ::AbstractVector) stopped calling kron(::AbstractMatrix, ::AbstractMatrix).

[oxinabox]

This is probably pretty fun, so I am am labelling this a good first issue.
The math is not complex, its just a little fiddy playing around with repeat and such