[mlir][sparse] Converting SparseTensorCOO to use standard C++-style iterators.

This differential comprises three related changes: (1) it gives SparseTensorCOO standard C++-style iterators; (2) it removes the old iterator stuff from SparseTensorCOO; and (3) it introduces SparseTensorIterator which behaves like the old SparseTensorCOO iterator stuff used to.

The SparseTensorIterator class is needed because the MLIR codegen cannot easily use the C++-style iterators (hence why SparseTensorCOO had the old iterator stuff).  Distinguishing SparseTensorIterator from SparseTensorCOO also helps improve API hygiene since these two classes are used for distinct purposes.  And having SparseTensorIterator as its own class enables changing the underlying implementation in the future, without needing to worry about updating all the codegen tests etc.

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D135485