[mlir][sparse] Combining `dimOrdering`+`higherOrdering` fields into `dimToLvl`

This is a major step along the way towards the new STEA design.  While a great deal of this patch is simple renaming, there are several significant changes as well.  I've done my best to ensure that this patch retains the previous behavior and error-conditions, even though those are at odds with the eventual intended semantics of the `dimToLvl` mapping.  Since the majority of the compiler does not yet support non-permutations, I've also added explicit assertions in places that previously had implicitly assumed it was dealing with permutations.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D151505