[mlir][sparse] Factoring out SparseTensorType class

This change adds a new `SparseTensorType` class for making the "dim" vs "lvl" distinction more overt, and for abstracting over the differences between sparse-tensors and dense-tensors.  In addition, this change also adds new type aliases `Dimension`, `Level`, and `FieldIndex` to make code more self-documenting.

Although the diff is very large, the majority of the changes are mechanical in nature (e.g., changing types to use the new aliases, updating variable names to match, etc).  Along the way I also made many variables `const` when they could be; the majority of which required only adding the keyword.  A few places had conditional definitions of these variables, requiring actual code changes; however, that was only done when the overall change was extremely local and easy to extract.  All these changes are included in the current patch only because it would be too onerous to split them off into a separate patch.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D143800