Add port comment node

[leonardt]

Allow attaching comment to expressions ports, needed to fix phanrahan/magma#543

[leonardt]

Also needed for ports.

[leonardt]

Changed it to just support ports for now since that's what is being used.

[rsetaluri]

Think the best way to do this is to have a templated class WithComment<T> that has a unique_ptr<T> node + std::string comment, then we get all the other nodes w/ comment for free. Then add a convenience function too.

template<typename T> class WithComment : public T {
  ...
  std::string comment;
  std::string get_comment() {
    return this->comment;
  }
}

template<typename T> std::unique_ptr<WithComment<T>> AddComment(std::unique_ptr<T> node, std::string comment) {
  return std::make_unique<WithComment<T>>(std::move(node), comment);
}

(could also avoid template<typename T> and just have it over the ABC for ast nodes.

If you don't want to do ^, I would at least suggest changing the name to PortWithComment.

[leonardt]

Clever design pattern, I've implemented this, but it requires that Nodes provide a "copy constructor" (quotes because it doesn't really create a clone of the unique pointer, instead it consumes it and uses it to recreate a new version of itself). This is required for implementing the base class constructor call, so WithComment<T> has a constructor of the form WithComment(std::unique_ptr<T> node, ...) and needs to call T(std::move(node)), so T needs to define a constructor that accepts a std::unique_ptr<T> and basically just recreates itself with the contents. Minor detail, but it needs to be added to all the types to be complete, however I've only added Port support for now since that's what's actually needed to unblock the downstream coreir issue.

[rsetaluri]

Ah I see, yeah because of the inheritance from T. Makes sense. Saved by the implementation being in the header 😁