Use Verilator macros for casting

[rkapuscik]

As noted here: antmicro/verilator#33 (comment)

In general, Verilator uses the VN_CAST macro to check and cast and the VN_IS macro to just check the type of the node.
At some point we will probably have to convert that to use the macros if we wish to be integrated with mainline Verilator