Add a preprocessor method/api to retrieve the existing macros

[mingodad]

We already have a method void printMacros(std::ostream &out) const; but there is no way to retrieve the defined macros in code to do something with then, maybe something like void macros() const -> std::vector<std::tuple<std::string_view, std::string_view>>;;

[robertoraggi]

Thanks for the suggestion. I plan to provide APIs to hooks into the preprocessor to get macros. Getting not only the value but also the point of definition is useful.