Rework private memory on the CPU

Created by: mwarusz

This PR changes @private memory on the CPU to return views to the underlying array with additional dimensions. The main rationale is that it makes passing private arrays to functions more straightforward. For example, currently @private vectors in our kernels are passed as @views f!(v[:], ...) which is a bit confusing to the uninitiated. Moreover, this change made is easy to enable scalar @private variables.

CC: @jkozdon