Support dense attributes in mod_arith constant folding and canonicalization

Currently it only supports scalars, but the `mod_arith.constant` op is elementwise and has uses now that involve dense attributes. Also cf. https://github.com/google/heir/pull/1758 since if/when that lands it will greatly simplify the work. Before that PR, one will need to ensure the attributes passed along the folding chain use an underlying integer type (`IntegerAttr` or `DenseIntElementsAttr`) corresponding to the modulus storage type. After that PR, one should be able to just use a `DenseIntElementsAttr` that is typed as a tensor of ModArith types.