Make storage_traits::TensorToArray pub

[AndrejOrsula]

Thank you for all the amazing work!

This tiny PR makes tensor::storage_traits::TensorToArray public as tensor::TensorToArray, similar to other storage_traits interfaces like tensor::TensorFromVec.

• This change enables usage of storage_traits::AsArray over generic storage_traits::Storage outside of dfdx as D: Storage<E> + TensorToArray<S, E>.
• Related to Make more functions/modules/traits public #796