Add "reflection" adjacent feature (#5002)

[ct-clmsn]

Implements reflection adjacent feature. code generator creates accessor methods and creates a look up table to get access to I/O ports. this will allow verilator to integrate with different simulation systems. see issue #5002

[ct-clmsn]

@wsnyder looks like std::variant is not supported in the C++ code generated by verilator. I'd suggest using a 3rd party variant implementation mpark's variant it's fairly popular and has good performance.

[wsnyder]

I'm not willing to add third-party dependencies for this feature, sorry. Please do without variant. Also I still have concerns about the support cost of this code.