[mlir][sparse] add a cursor to sparse storage scheme

This prepare a subsequent revision that will generalize
the insertion code generation. Similar to the support lib,
insertions become much easier to perform with some "cursor"
bookkeeping. Note that we, in the long run, could perhaps
avoid storing the "cursor" permanently and use some
retricted-scope solution (alloca?) instead. However,
that puts harder restrictions on insertion-chain operations,
so for now we follow the more straightforward approach.

Reviewed By: Peiming

Differential Revision: https://reviews.llvm.org/D136800