[Mir] Consider using direct computed indices for TraceAccess and PublicInputAccess #452
Description
This is a followup issue of #444.
Currently, the Mir values for PublicInputAccess and TraceAccess only take constant index and column references.
However, they can be accessed through computed indices. As a workaround, we currently wrap them in Accessor nodes (and handle the index of the accessor as an offset for the PublicInputAccess and TraceAccess).
However, to stay consistant, we should only have Accessor nodes with Index access types to index into a vector, not as offsets for scalar values into a vector.
Instead, PublicInputAccess and TraceAccess should reference node in the graph.