Just as we auto-compute expressions such as A + B in (A + B) + C, we can/should also auto-compute AmbiguousAssignOrExtract resulting from indexing such as v[idx] + w.

This requires that the object resulting from A[idx] know whether the return value is a scalar, vector, or matrix, and to have methods that match the output that will auto-compute. In other words, we need to expand AmbiguousAssignOrExtract into multiple classes such as ScalarIndexExpr, VectorIndexExpr, MatrixIndexExpr, and to populate each class with methods from grblas/_automethods.py.

We should also improve the repr of each of these new classes.

Notably, Assigner and Updater don't need updated for auto-compute.