Explore feasability of multi-dimensional arrays.

- How will this work with range expressions?
- How will we specify `arraysize` for multi-dimensional arrays?
- Will we be allowed to mix fixed array sizes with dynamic array sizes in a single multi-dimensional array?

Start with the IFEX README and the documentation at covesa.github.io/ifex/, then examine the existing array and range-expression definitions. Determine and document how multi-dimensional arrays would handle range expressions, arraysize declarations, and mixed fixed and dynamic dimensions; done means these questions have an agreed specification.

Written by the indexing model from the issue text.